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Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31,
	2019		2018		2017
Current:	(in thousands)
Federal	$	—	$	—	$	—
State	1		2		—
	1		2		—
Deferred:
Federal	(251)		(89)		—
State	(81)		(30)		—
	(332)		(119)		—
Benefit for income taxes	$	(331)	$	(117)	$	—

Schedule of Income Tax Rate Reconciliation

Reconciliation of the benefit for income taxes calculated at the statutory rate to our benefit for income taxes is as follows:


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31,
	2019		2018		2017
	(in thousands)
Tax benefit at federal statutory rate	$	(38,834)	$	(44,441)	$	—
State taxes, net of federal benefit	(12,951)		(10,652)		—
Stock-based compensation	2,037		3,629		—
Research tax credits	(1,714)		(708)		—
Write-off of in-process R&D	—		5,247		—
Change in fair value of convertible notes	—		4,454		—
Change in valuation allowance	49,989		41,916		—
Other	1,141		438		—
Benefit for incomes taxes	$	(331)	$	(117)	$	—

Schedule of Deferred Tax Assets and Liabilities

Significant components of our deferred tax assets and liabilities are as follows:


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31
	2019		2018		2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	54,018	$	16,437	$	—
Tax credit carryforwards	4,239		1,239		—
Intangibles	22,770		23,086		—
Accrued expenses	2,375		952		—
Lease liabilities	14,839		9,730		—
Stock based compensation	6,870		360		—
Other	—		—		—
Total deferred tax assets	105,111		51,804		—
Deferred tax liabilities:
Fixed assets	(361)		(531)		—
Right of use leased assets	(12,453)		(9,239)		—
Investments	(393)		(118)		—
Other	—		—		—
Total deferred tax liabilities	(13,207)		(9,888)		—
Net deferred tax assets	91,904		41,916		—
Valuation allowance	(91,904)		(41,916)		—
Net deferred tax assets	$	—	$	—	$	—

Summary of Operating Loss Carryforwards

The following table sets forth our federal and state NOL carryforwards and federal research and development tax credits as of December 31, 2019:


	Amount		Expiration
	(in thousands)
Net operating losses, federal	$	193,991	Indefinite
Net operating losses, federal	$	2	2037
Net operating losses, state	$	190,159	2037-2039
Tax credits, federal	$	4,037	2038-2039
Tax credits, state	$	3,832	Indefinite

Schedule of Reconciliation of Unrecognized Tax Benefits

We apply the provisions of ASC Topic 740 to account for uncertain income tax positions. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
	2019		2018		2017
	(in thousands)
Balance at beginning of the year:	920		—		—
Additions based on tax positions related to current year	2,228		920		—
Additions to tax position of prior year	—		—		—
Reductions to tax position of prior years	—		—		—
Lapse of the applicable statute of limitations	—		—		—
Balance at end of the year	$	3,148	$	920	$	—

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 175,126	$ 92,432
Short-term investments	355,407	366,952
Prepaid expenses and other current assets	14,043	8,598
Total current assets	544,576	467,982
Long-term investments	58,322	261,966
Operating lease right-of-use asset	44,495	33,015
Property and equipment, net	56,449	8,595
Intangible assets, net	151	754
Restricted cash	4,299	1,299
Other long-term assets	4,618	244
Equity method investment	4,892	0
Total assets	717,802	773,855
Current liabilities:
Accounts payable	9,250	12,338
Accrued and other current liabilities	23,829	17,121
Total current liabilities	33,079	29,459
Lease liability, noncurrent	51,349	34,456
Other long-term liabilities	4,351	6,776
Total liabilities	88,779	70,691
Commitments and Contingencies (Notes 6, 7 and 8)
Stockholders’ equity:
Preferred stock, $0.001 par value: 10,000,000 authorized as of December 31, 2019 and December 31, 2018, respectively; no shares were issued and outstanding as of December 31, 2019 and December 31, 2018	0	0
Common stock, $0.001 par value: 200,000,000 authorized as of December 31, 2019 and December 31, 2018; 124,267,358 and 121,482,671 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively	124	121
Additional paid-in capital	1,023,876	914,265
Accumulated deficit	(396,122)	(211,528)
Accumulated other comprehensive income	1,145	306
Total stockholders’ equity	629,023	703,164
Total liabilities and stockholders’ equity	$ 717,802	$ 773,855

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO	Series A Convertible Preferred Stock	Series A Convertible Preferred Stock Series A1 Convertible Preferred Shares	Subscriptions Receivables from Preferred Stockholders	Common Stock	Common Stock IPO	Notes Receivable from Common Stockholders	Additional Paid-in Capital	Additional Paid-in Capital IPO	Accumulated Deficit	Accumulated Other Comprehensive Income
Balance at Nov. 29, 2017	$ 0		$ 0		$ 0	$ 0		$ 0	$ 0		$ 0	$ 0
Balance (in shares) at Nov. 29, 2017			0			0
Issuance of common stock, net of issuance costs	5					$ 26					(21)
Issuance of common stock, net of issuance costs (in shares)						26,249,993
Notes receivable from common stockholders	(5)							(5)
Net loss and comprehensive loss	(2)										(2)
Net unrealized gain on available-for-sale investments, net of tax	0
Balance at Dec. 31, 2017	(2)		$ 0		0	$ 26		(5)	0		(23)	0
Balance (in shares) at Dec. 31, 2017			0			26,249,993
Balance at Nov. 29, 2017	0		$ 0		0	$ 0		0	0		0	0
Balance (in shares) at Nov. 29, 2017			0			0
Net loss and comprehensive loss	(396,100)
Balance at Dec. 31, 2019	$ 629,023		$ 0		0	$ 124		0	1,023,876		(396,122)	1,145
Balance (in shares) at Dec. 31, 2019	124,267,358		0			124,267,358
Balance at Dec. 31, 2017	$ (2)		$ 0		0	$ 26		(5)	0		(23)	0
Balance (in shares) at Dec. 31, 2017			0			26,249,993
Issuance of Series A convertible preferred shares at $35.06 per share, net of issuance costs of $635 and $84			$ 264,365	$ 34,917
Issuance of Series A convertible preferred shares at $35.06 per share, net of issuance costs of $635 and $84 (in shares)			7,557,990	998,225
Issuance of Series A-1 convertible preferred shares at $35.06 per share in connection with asset acquisition				$ 111,770
Issuance of Series A-1 convertible preferred shares at $35.06 per share in connection with asset acquisition (in shares)				3,187,772
Proceeds received from common stockholders for issuance of founders' stock at inception	5							5
Subscriptions receivable from preferred stockholders	(150,000)				(150,000)
Proceeds received from preferred stockholders	150,000				150,000
Issuance of common stock for early exercise of stock options	5					$ 5
Issuance of common stock for exercise of stock options (in shares)						5,020,580
Issuance of common stock, net of issuance costs		$ 343,329					$ 21			$ 343,308
Issuance of common stock, net of issuance costs (in shares)							20,700,000
Conversion of Series A convertible preferred stock	411,052		$ (411,052)			$ 62			410,990
Conversion of Series A convertible preferred stock (in shares)			(11,743,987)			61,655,922
Issuance of common stock upon conversion of convertible notes	141,410					$ 7			141,403
Issuance of common stock upon conversion of convertible stock (in shares)							7,856,176
Adjustment for fractional shares from forward stock split	(2)								(2)
Stock-based compensation	18,566								18,566
Net loss and comprehensive loss	(211,505)										(211,505)
Net unrealized gain on available-for-sale investments, net of tax	306											306
Balance at Dec. 31, 2018	$ 703,164		$ 0		0	$ 121		0	914,265		(211,528)	306
Balance (in shares) at Dec. 31, 2018	121,482,671		0			121,482,671
Issuance of common stock for early exercise of stock options	$ 2,959					$ 1			2,958
Issuance of common stock for exercise of stock options (in shares)						711,623
Employee stock purchase plan (in shares)						107,982
Employee stock purchase plan	1,783								1,783
Issuance of common stock, net of issuance costs	$ 54,219					$ 2			54,217
Issuance of common stock, net of issuance costs (in shares)	1,965,082					1,965,082
Vesting of early exercised common stock	$ 4,590								4,590
Stock-based compensation	46,063								46,063
Net loss and comprehensive loss	(184,594)										(184,594)
Net unrealized gain on available-for-sale investments, net of tax	839											839
Balance at Dec. 31, 2019	$ 629,023		$ 0		$ 0	$ 124		$ 0	$ 1,023,876		$ (396,122)	$ 1,145
Balance (in shares) at Dec. 31, 2019	124,267,358		0			124,267,358

Asset Acquisition - Schedule of Fair Value of Assets Acquired (Details) $ in Thousands	Apr. 30, 2018 USD ($)
Asset Acquisition [Line Items]
Property and equipment	$ 3,258
In Process Research and Development
Asset Acquisition [Line Items]
Total assets acquired	113,900
In Process Research and Development \| Anti-CD19 CAR T cell therapy
Asset Acquisition [Line Items]
Total assets acquired	103,936
In Process Research and Development \| Anti-BCMA CAR T cell therapy
Asset Acquisition [Line Items]
Total assets acquired	5,500
In Process Research and Development \| Assembled workforce
Asset Acquisition [Line Items]
Total assets acquired	$ 1,206

Net Loss and Net Loss Per Share - Schedule of Earnings Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended		25 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Numerator:
Net loss	$ (2)	$ (61,030)	$ (50,735)	$ (41,243)	$ (31,586)	$ (30,509)	$ (43,497)	$ (134,902)	$ (2,597)	$ (184,594)	$ (211,505)	$ (396,100)
Denominator:
Weighted average common shares outstanding (in shares)	26,249,993									101,061,149	28,948,386
Net loss per share, basic and diluted (in dollars per share)	$ 0.00	$ (0.58)	$ (0.50)	$ (0.41)	$ (0.32)	$ (0.37)	$ (10.71)	$ (43.82)	$ (0.10)	$ (1.83)	$ (7.31)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Description of Business and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Disclosure Text Block [Abstract]

Description of Business and Summary of Significant Accounting Policies

Allogene Therapeutics, Inc. (the Company or Allogene) was incorporated on November 30, 2017 in the State of Delaware and is headquartered in South San Francisco, California. Allogene is a clinical-stage immuno-oncology company pioneering the development and commercialization of genetically engineered allogeneic T cell therapies for the treatment of cancer. The Company is developing a pipeline of off-the-shelf T cell product candidates that are designed to target and kill cancer cells.

For the period from November 30, 2017 (inception) to December 31, 2017, the Company incurred $2,000 in start-up costs to establish the Company. Principal operations commenced in April 2018 when Allogene acquired certain assets from Pfizer Inc. (Pfizer) (see Note 6) and completed a Series A and A-1 preferred stock financing (see Note 11).

Public Offerings

In October 2018, the Company completed an initial public offering (IPO) of its common stock. In connection with its IPO, the Company issued and sold 20,700,000 shares of its common stock, which included 2,700,000 shares of its common stock issued pursuant to the over-allotment option granted to the underwriters, at a price to the public of $18.00 per share. As a result of the IPO, the Company received $343.3 million in net proceeds, after deducting underwriting discounts and commissions of $26.1 million and offering expenses of $3.2 million payable by the Company. At the closing of the IPO, 11,743,987 shares of outstanding convertible preferred stock were automatically converted into 61,655,922 shares of common stock and the 2018 Notes (see Note 10) were automatically converted into 7,856,176 shares of common stock. Following the IPO, there were no shares of convertible preferred stock or preferred stock outstanding.

In November 2019, the Company entered into a sales agreement with Cowen and Company, LLC (Cowen), under which the Company may from time to time issue and sell shares of its common stock through Cowen in at-the-market (ATM) offerings for an aggregate offering price of up to $250.0 million. The aggregate compensation payable to Cowen as the Company's sales agent equals up to 3.0% of the gross sales price of the shares sold through it pursuant to the sales agreement. Beginning November 2019 and through the year ended December 31, 2019, the Company sold an aggregate of 1,965,082 shares of common stock in ATM offerings resulting in net proceeds of $54.2 million, after deducting commissions and offering costs of $1.6 million.

Deferred Offering Costs

Offering costs, including legal, accounting and filing fees related to the IPO, were deferred and were offset against the offering proceeds upon the completion of the IPO. Upon the completion of the IPO in October 2018, $3.2 million of deferred offering costs were reclassified to additional paid in capital. Offering costs, including legal, accounting and filing fees related to the ATM offerings were incurred. Upon the issuance and sale of common stock in ATM offerings in November 2019, $0.5 million of deferred offering costs were reclassified to additional paid in capital. There were no deferred offering costs capitalized as of December 31, 2019 and 2018.

Forward Stock Split

On October 1, 2018, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a forward split of shares of the Company’s common stock on a 1-for-5.25 basis (the Forward Stock Split). In connection with the Forward Stock Split, the conversion ratio for the Company’s outstanding convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was increased in proportion to the Forward Stock Split. The par value of the common stock was not adjusted as a result of the Forward Stock Split. All references to common stock, options to purchase common stock, early exercised options, share data, per share data, convertible preferred stock (to the extent presented on an as-converted to common stock basis) and related information contained in these financial statements have been retrospectively adjusted to reflect the effect of the Forward Stock Split for all periods presented.

Need for Additional Capital

The Company has sustained operating losses and expects to continue to generate operating losses for the foreseeable future. The Company’s ultimate success depends on the outcome of its research and development activities as well as the ability

to commercialize the Company’s product candidates. The Company had cash and cash equivalents and investments of $588.9 million as of December 31, 2019. Since inception through December 31, 2019, the Company has incurred cumulative net losses of $396.1 million. Management expects to incur additional losses in the future to fund its operations and conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan.

The Company intends to raise such additional capital through the issuance of equity securities, debt financings or other sources in order to further implement its business plan. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its product candidates. The Company expects that its cash and cash equivalents and investments will be sufficient to fund its operations for at least the next 12 months from the date the Company’s Annual Report on Form 10-K is filed with the Securities and Exchange Commission (SEC).

Basis of Presentation

The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP).

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to the fair value of common stock, the fair value of stock options, the fair value of investments, the fair value of convertible notes payable upon conversion, income tax uncertainties, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances change. Actual results could differ from those estimates.

Concentration of Credit and other Risks and Uncertainties

Financial instruments, which potentially subject the Company to significant concentrations of credit risk, consist primarily of cash and cash equivalents and investments. The primary objectives for the Company’s investment portfolio are the preservation of capital and the maintenance of liquidity. The Company does not enter into any investment transaction for trading or speculative purposes.

The Company’s investment policy limits investments to certain types of instruments such as certificates of deposit, commercial paper, money market instruments, obligations issued by the U.S. government and U.S. government agencies as well as corporate debt securities, and places restrictions on maturities and concentration by type and issuer. The Company maintains cash balances in excess of amounts insured by the FDIC and concentrated within a limited number of financial institutions. The accounts are monitored by management and management believes that the financial institutions are financially sound, and, accordingly, minimal credit risk exists with respect to these financial institutions. As of December 31, 2019 and 2018, the Company has not experienced any credit losses in such accounts or investments.

The Company is subject to a number of risks common for early-stage biopharmaceutical companies including, but not limited to, dependency on the clinical and commercial success of its product candidates, ability to obtain regulatory approval of its product candidates, the need for substantial additional financing to achieve its goals, uncertainty of broad adoption of its approved products, if any, by physicians and patients, significant competition and untested manufacturing capabilities.

Segments

Operating segments are defined as components of an entity for which separate financial information is available and that is regularly reviewed by the Chief Operating Decision Maker (CODM) in deciding how to allocate resources to an individual segment and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company has determined it operates in a single operating segment and has one reportable segment.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts.

The Company has issued letters of credit under separate lease agreements which have been collateralized by restricted cash. This cash is classified as long-term restricted cash on the accompanying balance sheet based on the terms of the underlying leases.

Investments

Investments are available-for-sale and are carried at estimated fair value. The Company’s valuations of marketable securities are generally derived from independent pricing services based upon quoted prices in active markets for similar securities, with prices adjusted for yield and number of days to maturity, or based on industry models using data inputs, such as interest rates and prices that can be directly observed or corroborated in active markets. Management determines the appropriate classification of its investments in debt securities at the time of purchase and at the end of each reporting period. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from the balance sheet date are classified as current.

Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive income. The Company periodically evaluates whether declines in fair values of its available-for-sale securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the available-for-sale security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any available-for-sale securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in interest and other income, net. The cost of investments sold is based on the specific-identification method. Interest income on investments is included in interest and other income, net.

Fair Value Measurement

Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active.

Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Property and Equipment, Net

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to seven years. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in other expense.

The Company has determined the estimated life of assets to be as follows:


Laboratory equipment			5 years
Computer equipment and purchased software			3 - 5 years
Fixtures and furniture			7 years
Leasehold improvements			Shorter of lease term or useful life

Leases

The Company early adopted Accounting Standards Update (ASU) No. 2016-2, Leases on January 1, 2018. For its long-term operating leases, the Company recognizes a right-of-use asset and a lease liability on its balance sheets. The lease liability is determined as the present value of future lease payments using an estimated rate of interest that the Company would pay to borrow equivalent funds on a collateralized basis at the lease commencement date. The right-of-use asset is based on the liability adjusted for any prepaid or deferred rent. The lease term at the commencement date is determined by considering whether renewal options and termination options are reasonably assured of exercise.

Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses.

The Company elected to exclude from its balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease components for its long-term real-estate leases.

Equity Method Investments

The Company uses the equity method of accounting for equity investments in companies if the investment provides the ability to exercise significant influence, but not control, over operating and financial policies of the investee. The Company's proportionate share of the net income or loss of these companies is included in other expenses in the statement of operations. Judgment regarding the level of influence over each equity method investment includes considering key factors such as our ownership interest, representation on the board of directors, participation in policy-making decisions and material purchase and sale transactions.

The Company evaluates equity method investments for impairment whenever events or changes in circumstances indicate that the carrying amount of the investment might not be recoverable. Factors considered when reviewing an equity method investment for impairment include the length of time (duration) and the extent (severity) to which the fair value of the equity method investment has been less than cost, the investee’s financial condition and near-term prospects and the intent and ability to hold the investment for a period of time sufficient to allow for anticipated recovery. An impairment that is other-than-temporary is recognized in the period identified.

Variable Interest Entities

For entities in which the Company has variable interests, the Company focuses on identifying if one of the entities is the primary beneficiary through having the power to direct the activities that most significantly impact the variable interest entity’s economic performance and having the obligation to absorb losses or the right to receive benefits from the variable interest entity. If the Company is the primary beneficiary of a variable interest entity, the assets, liabilities, and results of operations of the variable interest entity will be included in the Company’s financial statements. For the year ended December 31, 2019, the Company did not consolidate any variable interest entities because the Company determined that it was not the primary beneficiary.

Accrued Research and Development Costs

The Company records accrued liabilities for estimated costs of research and development activities conducted by collaboration partners and third-party service providers, which include the conduct of preclinical studies and clinical trials, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced and includes these costs in accrued and other current liabilities on the balance sheets and within research and development expenses on the statements of operations and comprehensive loss.

The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its collaboration partners and third-party service providers. The Company makes significant judgments and estimates in determining the accrued liabilities balance at the end of each reporting period. As actual costs

become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception.

Income Taxes

Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and net losses, the net deferred tax assets have been fully offset by a valuation allowance.

The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of the provision for income taxes.

Stock-Based Compensation

The Company measures its stock-based awards granted to employees, consultants and directors based on the estimated fair values of the awards and recognizes the compensation over the requisite service period. The Company uses the Black-Scholes option-pricing model to estimate the fair value of its stock-based awards. Stock-based compensation is recognized using the straight-line method. As the stock compensation expense is based on awards ultimately expected to vest, it is reduced by forfeitures. The Company accounts for forfeitures as they occur.

Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share since the effects of potentially dilutive securities are antidilutive. Shares of common stock subject to repurchase are excluded from the weighted-average shares.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss. For the years ended December 31, 2019 and 2018 this was comprised of unrealized gains and losses, net of tax, on the Company’s investments. For the period from November 30, 2017 (inception) to December 31, 2017, comprehensive net loss was equal to net loss.

Definite-Lived Intangible Assets

Identifiable intangible assets consist of in-process research and development and workforce associated with the Pfizer asset acquisition. Intangible assets with finite lives are amortized over their estimated useful lives on a straight-line basis, generally two years. Acquired in-process research and development intangible assets with no alternative future use are charged to research and development expense when acquired. The straight-line method of amortization represents the Company’s best estimate of the distribution of the economic value of the identifiable intangible assets. Intangible assets are carried at cost less accumulated amortization. Amortization of intangible assets is included in research and development expenses.

Impairment of Long-Lived Assets

Long-lived assets are reviewed annually for impairment or whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount of an asset group to the future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. There were impairment losses related to equipment disposals of $0.2 million and zero for the years ended December 31, 2019 and 2018, respectively.

Research and Development Expenses

Research and development costs are expensed as incurred and consist of salaries and benefits, including associated stock-based compensation, and laboratory supplies and facility costs, as well as fees paid to other entities that conduct certain research and development activities on the Company’s behalf. Research and development expenses also include costs incurred for internal and sponsored collaborative research and development activities. Costs associated with co-development activities performed under the various license and collaboration agreements are included in research and development expenses.

Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed.

Research and development expenses for the year ended December 31, 2018 primarily consisted of acquired intangible assets pursuant to the Asset Contribution Agreement with Pfizer (see Note 6) as, at the time of acquisition of the asset, the technology was under development, was not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing, had not reached technical feasibility, or otherwise had no foreseeable alternative future use. For the year ended December 31, 2018, the Company recognized expense of $109.4 million related to the acquired intangible in-process research and development.

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Increase in valuation allowance	$ 50,000,000.0	$ 41,900,000
Tax benefit	300,000	100,000
Accrued interest and penalties	$ 0	$ 0

Balance Sheet Components - Schedule of Intangible Assets (Details) - Assembled workforce - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Research and Development Assets Acquired Other than Through Business Combination [Line Items]
Intangible assets, cost	$ 1,206	$ 1,206
Intangible assets, accumulated amortization	(1,055)	(452)
Intangible assets, carrying value	$ 151	$ 754

Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Leases

In August 2018, the Company entered into an operating lease agreement for new office and laboratory space which consists of approximately 68,000 square feet located in South San Francisco, California. The lease term is 127 months beginning August 2018 through February 2029 with an option to extend the term for another seven years which is not reasonably assured of exercise. The Company has the right to make tenant improvements, including the addition of laboratory space, with a lease incentive allowance of $5.1 million. The rent payments began on March 1, 2019 after an abatement period. In connection with the lease, the Company has maintained a letter of credit for the benefit of the landlord in the amount of $1.0 million. In connection with the lease, the Company recognized an operating lease right-of-use asset of $23.2 million and $24.6 million as of December 31, 2019 and 2018, respectively, and an aggregate lease liability of $30.9 million and $26.3 million, respectively, on the balance sheet. The remaining lease term is 9 years and 2 months, and the estimated incremental borrowing rate is 8.0%.

In October 2018, the Company entered into an operating lease agreement for new office and laboratory space which consists of approximately 14,943 square feet located in South San Francisco, California. The lease term is 124 months beginning November 2018 through February 2029, with an option to extend the term for another seven years which is not reasonably assured of exercise. The Company has the right to make tenant improvements, including the upgrading of current office and laboratory space with a lease incentive allowance of $0.8 million. Rent payments began in November 2018. In connection with the lease, the Company has maintained a letter of credit for the benefit of the landlord in the amount of $0.2 million. In connection with the lease, the Company recognized an operating lease right-of-use asset of $5.9 million and $6.2 million as of December 31, 2019 and 2018, respectively, and an aggregate lease liability of $6.2 million and $6.3 million, respectively, on the balance sheet. The remaining lease term is 9 years and 2 months, and the estimated incremental borrowing rate is 8.0%.

In December 2018, the Company entered into two operating leases for office space in New York and Los Angeles for approximately 4,358 and 1,293 square feet respectively. The Company recognized operating lease right-of-use assets of $1.7 million and $2.0 million for New York and $0.1 million and $0.2 million for Los Angeles as of December 31, 2019 and 2018, respectively. The Company recognized aggregate lease liabilities of $1.7 million and $2.0 million for New York and $0.2 million and $0.2 million for Los Angeles as of December 31, 2019 and 2018, respectively. The lease term for the New York operating lease is 6 years and 7 months, with no option for renewal. The lease term for the Los Angeles operating lease is 3 years with an option to extend the lease term for another 2 years which is not reasonably assured of exercise. There were no lease incentive allowances for either location. In connection with the New York lease, the Company maintained a letter of credit for the benefit of the landlord in the amount of $0.1 million. The remaining lease terms were 5 years and 6 months for New York and 1 year and 11 months for Los Angeles at December 31, 2019 and the estimated incremental borrowing rates applied were 8.0% and 7.5%, respectively.

In February 2019, the Company entered into a lease agreement for approximately 118,000 square feet of space to develop a cell therapy manufacturing facility in Newark, California. Upon certain conditions, the Company has two ten-year options to extend the lease, both of which are not reasonably assured of exercise. The Company is entitled to a tenant improvement allowance of $2.9 million for costs related to the design and construction of certain Company improvements. In connection with the lease, the Company will maintain a letter of credit for the benefit of the landlord in the amount of $3.0 million. The Company started accounting for this lease in October 2019 when tenant improvement work commenced, and recognized an operating lease right-of-use asset of $13.6 million as of December 31, 2019 and an aggregate lease liability of $14.0 million on its balance sheet. The lease term is 188 months, and the estimated incremental borrowing rate is 9.25%.

The undiscounted future non-cancellable lease payments under our operating leases as of December 31, 2019 is as follows:


Year ending December 31:	(in thousands)
2020	$	5,662
2021	7,632
2022	7,951
2023	8,203
2024 and thereafter	63,864
Total undiscounted lease payments	93,312
Less: Present value adjustment	(35,920)
Less: Tenant improvement allowance	(4,367)
Total	$	53,025

Rent expense for the Company’s operating leases was $5.9 million and $1.9 million for the years ended December 31, 2019 and 2018, respectively, net of lease incentives recognized. Rent expense for short-term leases was $2.4 million and $2.6 million for the years ended December 31, 2019 and 2018, respectively. There was a total commitment of zero and $1.6 million at December 31, 2019 and 2018, respectively, related to short-term leases. Variable lease payments for operating expenses were $1.1 million and immaterial for the years ended December 31, 2019 and 2018, respectively.

Certain lease agreements require the Company to return designated areas of leased space to its original condition upon termination of the lease agreement. At the inception of such leases, the Company records an asset retirement obligation and a corresponding capital asset in an amount equal to the estimated fair value of the obligation. To determine the fair value of the obligation, we estimate the cost for a third-party to perform the restoration work. In subsequent periods, for each asset retirement obligation, the Company records interest expense to accrete the asset retirement obligation liability to full value and depreciate each capitalized asset retirement obligation asset, both over the term of the associated lease agreement. Asset retirement obligations were $0.4 million and zero as of December 31, 2019 and 2018 respectively.

Related Party Transactions (Details)	1 Months Ended	12 Months Ended
Related Party Transactions (Details)	Aug. 31, 2018	Dec. 31, 2019 USD ($) director shares	Dec. 31, 2018 USD ($) ft² director shares	Sep. 30, 2019 ft²	Jun. 30, 2019	Feb. 28, 2019 ft²	Jan. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) \| shares		124,267,358	121,482,671
Operating lease right-of-use asset		$ 44,495,000	$ 33,015,000
New York
Related Party Transaction [Line Items]
Area of lease \| ft²			4,358
Operating lease term			6 years 7 months
Operating lease right-of-use asset		1,700,000	$ 2,000,000.0
Two River Consulting LLC
Related Party Transaction [Line Items]
Related party costs		$ 600,000	$ 600,000
Pfizer
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) \| shares		22,032,040	22,032,040
Number of members appointed to board of directors \| director		1	1
Payable to related party, current		$ 100,000	$ 5,700,000
Pfizer \| Transition services agreement
Related Party Transaction [Line Items]
Related party costs		4,500,000	10,100,000
Pfizer \| Research and development \| Transition services agreement \| Lab Supplies
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		1,400,000	10,400,000
TPG Capital – FO LLC \| Affiliated with President and Chief Executive Officer
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		0	$ 300,000
Ownership percentage					5.00%
Bellco \| Los Angeles California
Related Party Transaction [Line Items]
Area of lease \| ft²			1,293
Operating lease term			3 years
Operating lease right-of-use asset		100,000	$ 200,000
Bellco \| Consulting agreements
Related Party Transaction [Line Items]
Related party transaction monthly payment in arrears							$ 33,333.33	$ 26,250
Bellco \| Consulting agreements \| Maximum
Related Party Transaction [Line Items]
Related party transaction compensation percentage	60.00%
Bellco \| General and administrative \| Consulting agreements
Related Party Transaction [Line Items]
Related party costs		800,000	$ 500,000
ByHeart
Related Party Transaction [Line Items]
Sublease Income		$ 300,000
ByHeart \| New York
Related Party Transaction [Line Items]
Area of lease \| ft²				2,907		2,180

License and Collaboration Agreements (Details) € in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 EUR (€)	Nov. 01, 2019 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development	$ 0	$ 144,535,000	$ 151,860,000
Accrued and other current liabilities		23,829,000	17,121,000
Equity method investment		$ 4,892,000	0
License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percentage of development cost responsible for		60.00%
Pfizer \| Asset Contribution Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments, per target (in usd per share)		$ 840,000,000.0
Pfizer \| Asset Contribution Agreement \| United States
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments, per target (in usd per share)		30,000,000.0
Pfizer \| Asset Contribution Agreement \| European Union
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments, per target (in usd per share)		60,000,000.0
Pfizer \| Asset Contribution Agreement \| North America, Europe, Asia, Australia and Oceania
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payments per target		325,000,000.0
Cellectis \| Research Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum payments required per product against selected target		185,000,000.0
Research and development		0	400,000
Cellectis \| Clinical Development Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development		5,000,000.0
Cellectis \| Regulatory and Sales Milestone \| Maximum \| Research Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payable		2,800,000,000
Servier \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development		$ 7,300,000	4,200,000
Percentage of development cost responsible for		40.00%
Accrued and other current liabilities		$ 2,200,000	$ 4,200,000
Servier \| Regulatory Milestone \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payable		42,000,000
Servier \| Regulatory Milestone \| Maximum \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payments per target		137,500,000
Servier \| Sales Milestone \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payable		79,100,000		€ 70.5
Servier \| Sales Milestone \| Maximum \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payments per target		78,000,000.0
Notch Therapeutics, Inc. \| Research Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payable		7,250,000
Notch Therapeutics, Inc. \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development		10,000,000
Collaboration agreement, upfront payment					$ 10,000,000.0
Notch Therapeutics, Inc. \| Pre-Clinical Development Milestone \| Maximum \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payments per target		4,000,000.0
Notch Therapeutics, Inc. \| Clinical, Regulatory, and Commercial Milestone \| Maximum \| License and Collaboration Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate potential milestone payments per target		283,000,000.0
Notch Therapeutics, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Equity method investment		$ 4,900,000			$ 5,000,000.0
Ownership percentage		25.00%		25.00%	25.00%

Income Taxes - Schedule of Income Tax Reconciliation (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ 0	$ (38,834)	$ (44,441)
State taxes, net of federal benefit	0	(12,951)	(10,652)
Stock-based compensation	0	2,037	3,629
Research tax credits	0	(1,714)	(708)
Write-off of in-process R&D	0	0	5,247
Change in fair value of convertible notes	0	0	4,454
Change in valuation allowance	0	49,989	41,916
Other	0	1,141	438
Benefit for income taxes	$ 0	$ (331)	$ (117)

Net Loss and Net Loss Per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss and Net Loss Per Share

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,						Period From November 30, 2017 (Inception) to December 31,
	2019		2018		2017
Numerator:
Net loss	$	(184,594)	$	(211,505)	$	(2)
Denominator:
Weighted average common shares outstanding	101,061,149		28,948,386		26,249,993
Net loss per share, basic and diluted	$	(1.83)	$	(7.31)	$	—

Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive securities would have been anti-dilutive. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:


	Year Ended December 31,			Period From November 30, 2017 (Inception) to December 31,
	2019	2018	2017
Stock options to purchase common stock	9,190,522	7,235,545	—
Restricted stock units subject to vesting	1,941,155	—	—
Expected shares purchased under Employee Stock Purchase Plan	195,161	144,272	—
Founder shares subject to future vesting	13,629,803	19,687,487	—
Early exercised stock options subject to future vesting	2,992,290	5,020,580	—
Total	27,948,931	32,087,884	—

Investment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Maximum remaining contractual maturities of available-for-sale securities	2 years	3 years
Other than temporary impairment losses of available-for-sale securities	$ 0	$ 0

401(k) Plan (Details) - USD ($) $ in Millions	12 Months Ended
401(k) Plan (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Contribution expenses	$ 0.9	$ 0.4

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

For the years ended December 31, 2019 and 2018, the Company recorded income tax benefit due to the intraperiod tax allocation of deferred income taxes on unrealized gains on available for sale securities recorded in other comprehensive income. The Company has incurred net operating losses for all the periods presented. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements.

The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31,
	2019		2018		2017
Current:	(in thousands)
Federal	$	—	$	—	$	—
State	1		2		—
	1		2		—
Deferred:
Federal	(251)		(89)		—
State	(81)		(30)		—
	(332)		(119)		—
Benefit for income taxes	$	(331)	$	(117)	$	—

Reconciliation of the benefit for income taxes calculated at the statutory rate to our benefit for income taxes is as follows:


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31,
	2019		2018		2017
	(in thousands)
Tax benefit at federal statutory rate	$	(38,834)	$	(44,441)	$	—
State taxes, net of federal benefit	(12,951)		(10,652)		—
Stock-based compensation	2,037		3,629		—
Research tax credits	(1,714)		(708)		—
Write-off of in-process R&D	—		5,247		—
Change in fair value of convertible notes	—		4,454		—
Change in valuation allowance	49,989		41,916		—
Other	1,141		438		—
Benefit for incomes taxes	$	(331)	$	(117)	$	—

Deferred income taxes reflect the net tax effects of (a) temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and (b) operating losses and tax credit carryforwards.

Significant components of our deferred tax assets and liabilities are as follows:


	Year Ended December 31,						Period from November 30, 2017 (Inception) to December 31
	2019		2018		2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	54,018	$	16,437	$	—
Tax credit carryforwards	4,239		1,239		—
Intangibles	22,770		23,086		—
Accrued expenses	2,375		952		—
Lease liabilities	14,839		9,730		—
Stock based compensation	6,870		360		—
Other	—		—		—
Total deferred tax assets	105,111		51,804		—
Deferred tax liabilities:
Fixed assets	(361)		(531)		—
Right of use leased assets	(12,453)		(9,239)		—
Investments	(393)		(118)		—
Other	—		—		—
Total deferred tax liabilities	(13,207)		(9,888)		—
Net deferred tax assets	91,904		41,916		—
Valuation allowance	(91,904)		(41,916)		—
Net deferred tax assets	$	—	$	—	$	—

Realization of deferred tax assets is dependent upon future earnings, if any, the timing and amount of which are uncertain. Due to the lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. The valuation allowance increased by approximately $50.0 million and $41.9 million during the years ended December 31, 2019 and December 31, 2018, respectively.

The following table sets forth our federal and state NOL carryforwards and federal research and development tax credits as of December 31, 2019:


	Amount		Expiration
	(in thousands)
Net operating losses, federal	$	193,991	Indefinite
Net operating losses, federal	$	2	2037
Net operating losses, state	$	190,159	2037-2039
Tax credits, federal	$	4,037	2038-2039
Tax credits, state	$	3,832	Indefinite

Current federal and California tax laws include substantial restrictions on the utilization of NOLs and tax credit carryforwards in the event of an ownership change of a corporation. Accordingly, the Company's ability to utilize NOLs and tax credit carryforwards may be limited as a result of such ownership changes. Such a limitation could result in the expiration of carryforwards before they are utilized.

Income tax expense or benefit from continuing operations is generally determined without regard to other categories of earnings, such as discontinued operations and other comprehensive income. An exception is provided in ASC 740 when there is aggregate income from categories other than continuing operations and a loss from continuing operations in the current year. In this case, the tax benefit allocated to continuing operations is the amount by which the loss from continuing operations reduces the tax expenses recorded with respect to the other categories of earnings, even when a valuation allowance has been established against the deferred tax assets. In instances where a valuation allowance is established against current year losses, income from other sources, including gain from available-for-sale investments recorded as a component of other comprehensive

income, is considered when determining whether sufficient future taxable income exists to realize the deferred tax assets. For the years ended December 31, 2019 and 2018, the Company recorded a tax benefit of $0.3 million and $0.1 million, respectively, in other comprehensive income, related to available-for-sale securities.

We apply the provisions of ASC Topic 740 to account for uncertain income tax positions. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
	2019		2018		2017
	(in thousands)
Balance at beginning of the year:	920		—		—
Additions based on tax positions related to current year	2,228		920		—
Additions to tax position of prior year	—		—		—
Reductions to tax position of prior years	—		—		—
Lapse of the applicable statute of limitations	—		—		—
Balance at end of the year	$	3,148	$	920	$	—

It is the Company’s policy to include penalties and interest expense related to income taxes as a component of interest and other income, net, as necessary. As of December 31, 2019 and 2018, there were no accrued interest and penalties related to uncertain tax positions. The reversal of the uncertain tax benefits would not affect the effective tax rate to the extent that the Company continues to maintain a full valuation allowance against its deferred tax assets. Unrecognized tax benefits may change during the next 12 months for items that arise in the ordinary course of business. We are subject to examination by U.S. federal or state tax authorities for all years since inception.

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued compensation and related benefits	$ 9,560	$ 4,111
Accrued research and development expenses	4,833	7,808
Accrued property and equipment	3,575	0
Unvested shares liabilities	2,843	4,590
Other	3,018	612
Total accrued and other current liabilities	$ 23,829	$ 17,121

Leases - Summary of Undiscounted Future Non-Cancellable Lease Payments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2020	$ 5,662
2021	7,632
2022	7,951
2023	8,203
2024	63,864
Total undiscounted lease payments	93,312
Less: Present value adjustment	(35,920)
Less: Tenant improvement allowance	(4,367)
Total	$ 53,025

Equity Method Investment (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
Equity Method Investment (Details) - USD ($) $ in Thousands	Nov. 01, 2019	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity method investment			$ 4,892	$ 0
Income (loss) from investment		$ 0	(182)	$ 0
Notch Therapeutics, Inc.
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in Notch	$ 5,100
Transaction costs			$ 100
Ownership percentage	25.00%		25.00%
Equity method investment	$ 5,000		$ 4,900
Income (loss) from investment			$ 200

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 61,921	$ 9,643
Less: accumulated depreciation	(5,472)	(1,048)
Total property and equipment, net	56,449	8,595
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	13,117	5,534
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	29,924	15
Computers equipment and purchased software
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	3,726	1,327
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	2,764	64
Construction in Progress
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 12,390	$ 2,703

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted stock units subject to vesting $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, unvested, beginning balance (in shares) \| shares	0
Restricted stock units, granted (in shares) \| shares	1,982,855
Restricted stock units, vested (in shares) \| shares	(500)
Restricted stock units, forfeited (in shares) \| shares	(41,200)
Restricted stock units, unvested, ending balance (in shares) \| shares	1,941,155
Restricted stock units, vested and expected to vest (in shares) \| shares	1,941,155
Restricted stock units, weighted average grant date fair value, beginning balance (in dollars per share) \| $ / shares	$ 0
Restricted stock units, weighted average grant date fair value, grants (in dollars per share) \| $ / shares	27.45
Restricted stock units, weighted average grant date fair value, vested (in dollars per share) \| $ / shares	25.94
Restricted stock units, weighted average grant date fair value, forfeited (in dollars per share) \| $ / shares	27.48
Restricted stock units, weighted average grant date fair value, ending balance (in dollars per share) \| $ / shares	27.45
Restricted stock units, weighted average grant date fair value, vested and expected to vest (in dollars per share) \| $ / shares	$ 27.45
Restricted stock units, granted, weighted average remaining vesting life	1 year 11 months 8 days
Restricted stock units, unvested, weighted average remaining vesting life	1 year 11 months 23 days
Restricted stock units, vested and expected to vest, weighted average remaining vesting life	1 year 11 months 23 days
Restricted stock units, aggregate intrinsic value, beginning balance \| $	$ 0
Restricted stock units, aggregate intrinsic value, ending balance \| $	50,431
Restricted stock units, aggregate intrinsic value, vested and expected to vest \| $	$ 50,431

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of the year		$ 920	$ 0
Additions based on tax positions related to current year	$ 0	2,228	920
Additions to tax position of prior year	0	0	0
Reductions to tax position of prior years	0	0	0
Lapse of the applicable statute of limitations	0	0	0
Balance at end of the year	$ 0	$ 3,148	$ 920

Convertible Notes Payable (2018 Notes)	12 Months Ended
Convertible Notes Payable (2018 Notes)	Dec. 31, 2019
Debt Disclosure [Abstract]
Convertible Notes Payable (2018 Notes)	Convertible Notes Payable (2018 Notes) In September 2018, the Company entered into a note purchase agreement pursuant to which it sold and issued an aggregate of $120.2 million in convertible promissory notes (convertible notes payable or 2018 Notes) and received net cash proceeds of $116.8 million. On issuance, the fair value of the 2018 Notes was determined to be equal to $120.2 million, which is the principal amount of the 2018 Notes. The 2018 Notes did not accrue interest. The 2018 Notes were settled in 7,856,176 shares of common stock in connection with the closing of the Company’s IPO (see Note 1) at a settlement price equal to 85% of the IPO price per share. On issuance, the Company elected to account for the 2018 Notes at fair value with any changes in estimated fair value being recognized through the statements of operations and comprehensive loss until the 2018 Notes settled. The fair value of the 2018 Notes was determined to be $141.4 million upon settlement. For the years ended December 31, 2019 and 2018, the Company recognized zero and $21.2 million, respectively, of expense in the accompanying statements of operations and comprehensive loss for the change in fair value of the 2018 Notes. On issuance, total debt issuance costs of $3.4 million were expensed and recognized as interest expense in the accompanying statements of operations and comprehensive loss.

Label	Element	Value
Unrecognized Tax Benefits	us-gaap_UnrecognizedTaxBenefits	$ 0

Balance Sheet Components - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Components [Line Items]
Depreciation and amortization expense		$ 4,600,000	$ 1,000,000.0
Disposal of property, plant, and equipment		200,000	0
Amortization expense	$ 0	$ 603,000	452,000
Assembled workforce
Balance Sheet Components [Line Items]
Intangible assets, weighted-average remaining amortization period		3 months 3 days
Amortization expense		$ 600,000	$ 500,000

Asset Acquisition

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Asset Acquisition

In April 2018, the Company entered into an Asset Contribution Agreement (the Pfizer Agreement) with Pfizer pursuant to which the Company acquired certain assets, including certain contracts described in Note 7, and intellectual property for the development and administration of chimeric antigen receptor (CAR) T cells for the treatment of cancer.

As consideration for the purchased assets, the Company issued Pfizer 3,187,772 shares of its Series A-1 convertible preferred stock with an estimated fair value of $111.8 million or $35.06 per share. The Company also incurred $2.1 million of direct expenses related to the asset acquisition, bringing the total consideration to $113.9 million. The fair value of the Series A-1 convertible preferred stock was established using the price per share paid by third-party investors in the concurrent closing of the Series A and A-1 convertible preferred stock financing at $35.06 per share as well as the price per share paid by Pfizer to purchase additional shares of Series A-1 convertible preferred stock at $35.06 per share at the same time and at the same price per share as the rest of Series A and A-1 shares sold in such financing (see Note 11 for additional details). The Series A-1 convertible preferred shares issued to Pfizer had the same rights, preferences and privileges as the Series A convertible preferred shares issued to the third-party investors.

The Company accounted for the transaction as an asset acquisition as substantially all of the estimated fair value of the gross assets acquired was concentrated in a single identified asset, anti-CD19 CAR T cell therapy, thus satisfying the requirements of the screen test in ASU 2017-1. The assets acquired in the transaction were measured based on the fair value of the Series A-1 convertible preferred stock issued to Pfizer and direct transaction costs of $2.1 million, as the fair value of the

equity given was more readily determinable than the fair value of the assets received. The following table summarizes the fair value of assets acquired (in thousands):


Property and equipment	$	3,258
In-process research and development (IPR&D):
Anti-CD19 CAR T cell therapy	103,936
Anti-BCMA CAR T cell therapy	5,500
Assembled workforce	1,206
Total assets acquired	$	113,900

The estimated fair values of anti-CD19 CAR T cell therapy and anti-BCMA CAR T cell therapy were determined using a risk-adjusted discounted cash flow approach, which used the present value of the direct cash flows expected to be generated by anti-CD19 CAR T cell therapy and anti-BCMA CAR T cell therapy during their estimated economic lives, net of returns on contributory assets such as working capital, property and equipment, and the assembled workforce. The discount rate of 16.5% was based on rates of return available from alternative investments of similar type and quality as of the valuation date. The remaining IPR&D targets were determined to be more conceptual in nature with nominal value being attributed to them. The estimate of the fair value of the assembled workforce was determined using a replacement cost approach, based on the estimated cost of recruiting and training an equivalent workforce as of the acquisition date.

The amount allocated to intangible IPR&D assets was charged to research and development expenses as these assets had no alternative future use at the time of the acquisition transaction. The remaining intangible asset relates to the assembled workforce which was capitalized and is being amortized over its estimated economic life of two years to research and development expenses.

In addition, under the terms of the Pfizer Agreement, the Company was also required to make milestone payments to Pfizer of $30.0 million or $60.0 million per target (depending on the target, and up to $840.0 million in the aggregate for all targets) upon successful completion of certain regulatory and sales milestones for certain targets covered by the Pfizer Agreement. No milestone payments were made or became due in the years ended December 31, 2019 and 2018. These contingent payments were not part of the consideration for the purchased assets.

As part of the asset acquisition, the Company also assumed licensing agreements Pfizer had entered into with two third-party entities holding certain intellectual property. Both agreements cover use of the intellectual property held by the parties and certain research collaboration activities. See Note 7 for additional details on these agreements.

Under the Pfizer Agreement, the Company was required to use commercially reasonable efforts to develop and seek regulatory approval in and for the United States and the European Union for certain products covered by the Pfizer Agreement and to commercialize each product covered by the Pfizer Agreement in the applicable royalty territory in which regulatory approval for such product has been obtained.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions As of December 31, 2019 and 2018, Pfizer held 22,032,040 shares of Common Stock and had appointed one member to the Company’s Board of Directors. In April 2018, the Company and Pfizer entered into a transition services agreement (the Pfizer TSA) for Pfizer to provide professional services to the Company related to research and development, project management, and other administrative functions. In September 2019, the Company and Pfizer terminated the Pfizer TSA. For the years ended December 31, 2019 and 2018, the costs incurred under the Pfizer TSA were $4.5 million and $10.1 million, respectively. The Company also purchased certain lab supplies and services from Pfizer in connection with its research and development activities. For the years ended December 31, 2019 and 2018, total lab supplies and services purchased from Pfizer were $1.4 million and $10.4 million, respectively. As of December 31, 2019 and 2018, the Company had amounts payable to Pfizer of $0.1 million and $5.7 million, respectively, which were recorded in the accompanying balance sheet. Consulting Agreements In June 2018, the Company entered into a services agreement with Two River Consulting LLC (Two River) a firm affiliated with the Company’s President and Chief Executive Officer, the Company’s Executive Chairman of the board of directors, and a director of the Company to provide various managerial, administrative, accounting and financial services to the Company. The costs incurred for services provided under this agreement were $0.6 million for the years ended December 31, 2019 and 2018. In June 2018 the Company entered into a consulting services agreement with TPG Capital – FO LLC (TPG FO) a firm affiliated with a beneficial owner of more than 5% of the Company’s capital stock. The costs incurred for services performed under this agreement were zero and $0.3 million for the years ended December 31, 2019 and 2018, respectively. In August 2018, the Company entered into a consulting agreement with Bellco Capital LLC (Bellco). The Company’s executive chairman, Arie Belldegrun, M.D., FACS, is the Chairman and an owner of Bellco. Pursuant to the consulting agreement, Bellco provides certain services for the Company, which are performed by Dr. Belldegrun and include without limitation, providing advice and analysis with respect to the Company’s business, business strategy and potential opportunities in the field of allogeneic CAR T cell therapy and any other aspect of the CAR T cell therapy business as the Company may agree. In consideration for these services, the Company paid Bellco $26,250 per month in arrears commencing June 2018. Beginning January 2019, the Company paid Bellco $33,333.33 per month in arrears and, at the Company’s discretion, may pay Bellco an annual performance award in an amount up to 60% of the aggregate compensation payable to Bellco in a calendar year. The Company also reimburses Bellco for out of pocket expenses incurred in performing the services. The costs incurred for services provided, bonus and out-of-pocket expenses incurred under this consulting agreement were $0.8 million and $0.5 million for the years ended December 31, 2019 and 2018, respectively. Sublease Agreements In December 2018, the Company entered into a sublease with Bellco for 1,293 square feet of office space in Los Angeles California for a three year term. The total right of use asset and associated liability recorded related to this related party lease was $0.1 million and $0.2 million at December 31, 2019 and 2018, respectively. In February 2019, the Company subleased 2,180 square feet of its office space in New York, New York, to ByHeart, Inc., formerly known as Second Science, Inc. (ByHeart). ByHeart is a development-stage infant formula company. Two of the Company’s board members have beneficial ownership in ByHeart and one serves on the board of directors of ByHeart. In September 2019, the Company entered into an amendment to the sublease agreement and increased the subleased space to 2,907 square feet. Sublease income for the year ended December 31, 2019 was $0.3 million and was recognized as other income.

License and Collaboration Agreements	12 Months Ended
License and Collaboration Agreements	Dec. 31, 2019
Research and Development [Abstract]
License and Collaboration Agreements	License and Collaboration Agreements Asset Contribution Agreement with Pfizer In connection with the Pfizer Agreement (see Note 6), the Company is required to make milestone payments upon successful completion of regulatory and sales milestones on a target-by-target basis for the targets including CD19 and B-cell maturation antigen (BCMA), covered by the Pfizer Agreement. The aggregate potential milestone payments upon successful completion of various regulatory milestones in the United States and the European Union are $30.0 million or $60.0 million, depending on the target, with aggregate potential regulatory and development milestones of up to $840.0 million, provided that the Company is not obligated to pay a milestone for regulatory approval in the European Union for an anti-CD19 allogeneic CAR T cell product, to the extent Servier has commercial rights to such territory. The aggregate potential milestone payments upon reaching certain annual net sales thresholds in North America, Europe, Asia, Australia and Oceania (the Territory) for a certain number of targets covered by the Pfizer Agreement are $325.0 million per target. The sales milestones in the foregoing sentence are payable on a country-by-country basis until the last to expire of any Pfizer Royalty Term, as described below, for any product in such country in the Territory. In October 2019, the Territory was expanded to all countries in the world. No milestone or royalty payments were made in the years ended December 31, 2019 and 2018. Pfizer is also eligible to receive, on a product-by-product and country-by-country basis, royalties in single-digit percentages on annual net sales for products covered by the Pfizer Agreement or that use certain Pfizer intellectual property and for which an IND is first filed on or before April 6, 2023. The Company’s royalty obligation with respect to a given product in a given country begins upon the first sale of such product in such country and ends on the later of (i) expiration of the last claim of any applicable patent or (ii) 12 years from the first sale of such product in such country. Research Collaboration and License Agreement with Cellectis As part of the Pfizer Agreement (see Note 6), Pfizer assigned to the Company a Research Collaboration and License Agreement (the Original Cellectis Agreement) with Cellectis S.A. (Cellectis). On March 8, 2019, the Company entered into a License Agreement (the Cellectis Agreement) with Cellectis. In connection with the execution of the Cellectis Agreement, on March 8, 2019, the Company and Cellectis also entered into a letter agreement (the Letter Agreement), pursuant to which the Company and Cellectis agreed to terminate the Original Cellectis Agreement. The Original Cellectis Agreement included a research collaboration to conduct discovery and pre-clinical development activities to generate CAR T cells directed at targets selected by each party, which was completed in June 2018. Pursuant to the Cellectis Agreement, Cellectis granted to the Company an exclusive, worldwide, royalty-bearing license, on a target-by-target basis, with sublicensing rights under certain conditions, under certain of Cellectis’s intellectual property, including its TALEN and electroporation technology, to make, use, sell, import, and otherwise exploit and commercialize CAR T products directed at certain targets, including BCMA, FLT3, DLL3 and CD70 (the Allogene Targets), for human oncologic therapeutic, diagnostic, prophylactic and prognostic purposes. In addition, certain Cellectis intellectual property rights granted by Cellectis to the Company and to Servier pursuant to the Exclusive License and Collaboration Agreement by and between Servier and Pfizer, dated October 30, 2016, which Pfizer assigned to the Company in April 2018, will survive the termination of the Original Cellectis Agreement. Pursuant to the Cellectis Agreement, the Company granted Cellectis a non-exclusive, worldwide, royalty-free, perpetual and irrevocable license, with sublicensing rights under certain conditions, under certain of the Company's intellectual property, to make, use, sell, import and otherwise commercialize CAR T products directed at certain targets (the Cellectis Targets). The Cellectis Agreement provides for development and sales milestone payments by the Company of up to $185.0 million per product that is directed against an Allogene Target, with aggregate potential development and sales milestone payments totaling up to $2.8 billion. Cellectis is also eligible to receive tiered royalties on annual worldwide net sales of any products that are commercialized by the Company that contain or incorporate, are made using or are claimed or covered by, Cellectis intellectual property licensed to the Company under the Cellectis Agreement (the Allogene Products), at rates in the high single-digit percentages. Such royalties may be reduced, on a licensed product-by-licensed product and country-by-country basis, for generic entry and for payments due under licenses of third party patents. Pursuant to the Cellectis Agreement, and subject to certain exceptions, the Company is required to indemnify Cellectis against all third party claims related to the development, manufacturing, commercialization or use of any Allogene Product or arising out of the Company’s material breach of the representations, warranties or covenants set forth in the Cellectis Agreement, and Cellectis is required, subject to certain exceptions, to indemnify the Company against all third party claims related to the development, manufacturing, commercialization or use of CAR T products directed at Cellectis Targets or arising out of Cellectis’s material breach of the representations, warranties or covenants set forth in the Cellectis Agreement. The royalties are payable, on a licensed product-by-licensed product and country-by-country basis, until the later of (i) the expiration of the last to expire of the licensed patents covering such product; (ii) the loss of regulatory exclusivity afforded such product in such country, and (iii) the tenth anniversary of the date of the first commercial sale of such product in such country; however, in no event shall such royalties be payable, with respect to a particular licensed product, past the twentieth anniversary of the first commercial sale for such product. Depending on the Cellectis Target, the Company has a right of first refusal or right of first negotiation to purchase or license from Cellectis rights to develop and commercialize products against such Cellectis Targets. Under the Cellectis Agreement, the Company has certain diligence obligations to progress the development of CAR T product candidates and to commercialize one CAR T product per Allogene Target in one major market country where the Company has received regulatory approval. If the Company materially breaches any of its diligence obligations and fails to cure within 90 days, then with respect to certain targets, such target will cease to be an Allogene Target and instead will become a Cellectis Target. Unless earlier terminated in accordance with its terms, the Cellectis Agreement will expire on a product-by-product and country-by-country basis, upon expiration of all royalty payment obligations with respect to such licensed product in such country. The Company has the right to terminate the Cellectis Agreement at will upon 60 days’ prior written notice, either in its entirety or on a target-by-target basis. Either party may terminate the Cellectis Agreement, in its entirety or on a target-by-target basis, upon 90 days’ prior written notice in the event of the other party’s uncured material breach. The Cellectis Agreement may also be terminated by the Company upon written notice at any time in the event that Cellectis becomes bankrupt or insolvent or upon written notice within 60 days of a consummation of a change of control of Cellectis. All costs the Company incurred in connection with this agreement were recognized as research and development expenses. For the year ended December 31, 2019, $5.0 million of costs were incurred related to the achievement of a clinical development milestone under this agreement, which was recognized in research and development expenses in the statement of operations. No clinical development milestones were achieved for the year ended December 31, 2018. For the years ended December 31, 2019 and 2018, zero and $0.4 million, respectively, of costs have been incurred associated with research services performed by Cellectis. License and Collaboration Agreement with Servier As part of the Pfizer Agreement (see Note 6), Pfizer assigned to the Company an Exclusive License and Collaboration Agreement (the Servier Agreement), with Les Laboratoires Servier SAS and Institut de Recherches Internationales Servier SAS (collectively, Servier) to develop, manufacture and commercialize certain allogeneic anti-CD19 CAR T cell product candidates, including UCART19, in the United States with the option to obtain the rights over additional anti-CD19 product candidates and for allogeneic CAR T cell product candidates directed against one additional target. In October 2019, the Company agreed to waive its rights to the one additional target. Under the Servier Agreement, the Company has an exclusive license to develop, manufacture and commercialize UCART19 in the field of anti-tumor adoptive immunotherapy in the United States, with an exclusive option to obtain the same rights for additional product candidates in the United States and, if Servier does not elect to pursue development or commercialization of those product candidates in certain markets outside of the United States pursuant to its license, outside of the United States as well. The Company is not required to make any additional payments to Servier to exercise an option. If the Company opts-in to another product candidate, Servier has the right to obtain rights to such product candidate outside the United States and to share development costs for such product candidate. Under the Servier Agreement, the Company is required to use commercially reasonable efforts to develop and obtain marketing approval in the United States in the field of anti-tumor adoptive immunotherapy for at least one product directed against CD19, and Servier is required to use commercially reasonable efforts to develop and obtain marketing approval in the European Union, and one other country in a group of specified countries outside of the European Union and the United States, in the field of anti-tumor adoptive immunotherapy for at least one allogeneic adaptive T cell product directed against a certain Company-selected target. For product candidates that the Company is co-developing with Servier, including UCART19, ALLO-501 and ALLO-501A, the Company is responsible for 60% of the specified development costs and Servier is responsible for the remaining 40% of the specified development costs under the applicable global research and development plan. Subject to certain restrictions, each party has the right to conduct activities that are specific to its territory outside the global research and development plan at such party’s sole expense. In addition, each party is solely responsible for commercialization activities in its territory at such party’s sole expense. The Company is required to make milestone payments to Servier upon successful completion of regulatory and sales milestones. The Servier Agreement provides for aggregate potential payments by the Company to Servier of up to $137.5 million upon successful completion of various regulatory milestones, and aggregate potential payments by the Company to Servier of up to $78.0 million upon successful completion of various sales milestones. Similarly, Servier is required to make milestone payments upon successful completion of regulatory and sales milestones for products directed at the Allogene-target covered by the Servier Agreement that achieves such milestones. The total potential payments that Servier is obligated to make to the Company under the Servier Agreement upon successful completion of regulatory and sales milestones are $42 million and €70.5 million ($79.1 million), respectively. The foregoing milestones are subject to certain adjustments if the Company obtains rights for certain products outside of the United States upon Servier’s election not to pursue such rights. Each party is also eligible to receive tiered royalties on annual net sales in countries within the paying party’s respective territory of any licensed products that are commercialized by such party that are directed at the targets licensed by such party under the Servier Agreement. The royalty rates are in a range from the low tens to the high teen percentages. Such royalties may be reduced for interchangeable drug entry, expiration of patent rights and amounts paid pursuant to licenses of third-party patents. The royalty obligation for each party with respect to a given licensed product in a given country in each party’s respective territory (the Servier Royalty Term) begins upon the first commercial sale of such product in such country and ends after a defined number of years. Unless earlier terminated in accordance with the Servier Agreement, the Servier Agreement will continue, on a licensed product-by-licensed product and country-by-country basis, until the Servier Royalty Term with respect to the sale of such licensed product in such country expires. For the years ended December 31, 2019 and 2018, the Company recorded $7.3 million and $4.2 million, respectively, of costs incurred under the cost-sharing terms of the Servier Agreement as research and development expenses. As of December 31, 2019 and 2018, amounts due to Servier of $2.2 million and $4.2 million were recorded in accrued and other current liabilities in the accompanying balance sheets. Research Collaboration and License Agreement with Notch On November 1, 2019, the Company entered into a Collaboration and License Agreement (the Notch Agreement) with Notch Therapeutics Inc. (Notch), pursuant to which Notch granted to Allogene an exclusive, worldwide, royalty-bearing, sublicensable license under certain of Notch’s intellectual property to develop, make, use, sell, import, and otherwise commercialize therapeutic gene-edited T cell and/or natural killer (NK) cell products from induced pluripotent stem cells directed at certain CAR targets for initial application in non-Hodgkin lymphoma, acute lymphoblastic leukemia and multiple myeloma. In addition, Notch has granted Allogene an option to add certain specified targets to its exclusive license in exchange for an agreed per-target option fee. The Notch Agreement includes a research collaboration to conduct research and pre-clinical development activities to generate engineered cells directed to Allogene’s exclusive targets, which will be conducted in accordance with an agreed research plan and budget under the oversight of a joint development committee. Allogene will reimburse Notch’s costs incurred in accordance with such plan and budget. The term of the research collaboration will expire upon the earlier of (i) the fifth anniversary of the date of the Notch Agreement, (ii) at Allogene’s election, following the joint development committee’s determination that for each exclusive target, Notch has met certain success criteria, or (iii) the joint development committee’s determination that the research collaboration cannot be reasonably pursued against any exclusive target due to technical infeasibility or safety issues. In connection with the execution of the Notch Agreement, Allogene made an upfront payment to Notch of $10.0 million in return for a license to access Notch's technology in order to conduct research pursuant to the Notch Agreement. The Company recognized a research and development expense of $10 million during the year to December 31, 2019 as the license had no foreseeable alternative future use. In addition, Allogene made a $5.0 million investment in Notch’s series seed convertible preferred stock, resulting in Allogene having a 25% ownership interest in Notch’s outstanding capital stock on a fully diluted basis immediately following the investment. In connection with this investment, David Chang, M.D., Ph.D., the Company's President, Chief Executive Officer and Board member, was appointed to Notch’s board of directors. Under the Notch Agreement, Notch will be eligible to receive up to $7.25 million upon achieving certain agreed research milestones, up to $4.0 million per exclusive target upon achieving certain pre-clinical development milestones, and up to $283.0 million per exclusive target and cell type (i.e., T cell or NK cell) upon achieving certain clinical, regulatory and commercial milestones. Notch is also entitled to receive tiered royalties in the mid to high single digit range on Allogene’s sales of licensed products, subject to certain reductions, for a term, on a country-by-country and product-by-product basis, commencing on first commercial sale of such product in such country and continuing until the latest of (i) the date upon which there is no valid claim of the licensed patents in such country of sale that covers such product, (ii) the expiration of applicable data or other regulatory exclusivity in such country of sale or (iii) a defined period from the first commercial sale of such product in such country. The terms of the Notch Agreement will continue on a product-by-product and country-by-country basis until Allogene’s payment obligations with respect to such product in such country have expired. Following such expiration, Allogene’s license with respect to such product and country shall be perpetual, irrevocable, fully paid up and royalty-free. Allogene may terminate the Collaboration Agreement in whole or on a product-by-product basis upon ninety days’ prior written notice to Notch. Either party may also terminate the Collaboration Agreement with written notice upon material breach by the other party, if such breach has not been cured within a defined period of receiving such notice, or in the event of the other party’s insolvency. The Company has determined that Notch is a variable interest entity as of December 31, 2019. The Company does not have the power to direct the activities which most significantly affect Notch's economic performance. Accordingly, for the year ended December 31, 2019, the Company did not consolidate Notch because the Company determined that it was not the primary beneficiary.

Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended		25 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Selected Quarterly Financial Information [Abstract]
Loss from operations	$ (2)	$ (64,575)	$ (55,011)	$ (45,961)	$ (36,461)	$ (33,046)	$ (22,187)	$ (135,012)	$ (2,597)	$ (202,008)	$ (192,842)
Net loss	$ (2)	$ (61,030)	$ (50,735)	$ (41,243)	$ (31,586)	$ (30,509)	$ (43,497)	$ (134,902)	$ (2,597)	$ (184,594)	$ (211,505)	$ (396,100)
Net loss per share, basic and diluted (in dollars per share)	$ 0.00	$ (0.58)	$ (0.50)	$ (0.41)	$ (0.32)	$ (0.37)	$ (10.71)	$ (43.82)	$ (0.10)	$ (1.83)	$ (7.31)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Leases - Additional Information (Details)	1 Months Ended	12 Months Ended
Leases - Additional Information (Details)	Feb. 28, 2019 USD ($) ft² option	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) ft² operatingLease	Oct. 31, 2018 USD ($) ft²	Aug. 31, 2018 USD ($) ft²
Operating Leased Assets [Line Items]
Allowance for tenant improvements		$ 4,367,000
Operating lease right-of-use asset		44,495,000	$ 33,015,000
Operating lease liability		53,025,000
Number of operating Leases for office space \| operatingLease			2
Rent expense		5,900,000	$ 1,900,000
Rent expense for short-term leases		2,400,000	2,600,000
Short tem lease total commitment		0	1,600,000
Variable lease payments		1,100,000
Asset retirement obligation		400,000	$ 0
New York
Operating Leased Assets [Line Items]
Area of lease \| ft²			4,358
Operating lease term			6 years 7 months
Letter of credit		100,000
Operating lease right-of-use asset		1,700,000	$ 2,000,000.0
Operating lease liability		$ 1,700,000	$ 2,000,000.0
Remaining lease term		5 years 6 months
Estimated incremental borrowing rate of lease		8.00%
Los Angeles
Operating Leased Assets [Line Items]
Area of lease \| ft²			1,293
Operating lease term			3 years
Operating lease, extended term		2 years
Operating lease right-of-use asset		$ 100,000	$ 200,000
Operating lease liability		$ 200,000	200,000
Remaining lease term		1 year 11 months
Estimated incremental borrowing rate of lease		7.50%
Newark
Operating Leased Assets [Line Items]
Area of lease \| ft²	118,000
Operating lease term	188 months
Operating lease, options to extend term \| option	2
Operating lease, extended term	10 years
Allowance for tenant improvements		$ 2,900,000
Letter of credit	$ 3,000,000.0
Operating lease right-of-use asset		13,600,000
Operating lease liability		$ 14,000,000.0
Estimated incremental borrowing rate of lease		9.25%
127 Months Lease Term
Operating Leased Assets [Line Items]
Area of lease \| ft²					68,000
Operating lease term					127 months
Operating lease, extended term					7 years
Allowance for tenant improvements					$ 5,100,000
Letter of credit					$ 1,000,000.0
Operating lease right-of-use asset		$ 23,200,000	24,600,000
Operating lease liability		$ 30,900,000	26,300,000
Remaining lease term		9 years 2 months
Estimated incremental borrowing rate of lease		8.00%
124 Months Lease Term
Operating Leased Assets [Line Items]
Area of lease \| ft²				14,943
Operating lease term				124 months
Operating lease, extended term				7 years
Allowance for tenant improvements				$ 800,000
Letter of credit				$ 200,000
Operating lease right-of-use asset		$ 5,900,000	6,200,000
Operating lease liability		$ 6,200,000	$ 6,300,000
Remaining lease term		9 years 2 months
Estimated incremental borrowing rate of lease		8.00%

Description of Business and Summary of Significant Accounting Policies - Schedule of Estimated Life of Assets (Details)	12 Months Ended
	Dec. 31, 2019
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	three
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	seven
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	5 years
Computer Equipment and purchased software \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	P3Y
Computer Equipment and purchased software \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	P5Y
Fixtures and Furniture
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	7 years

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Stock Option Activity Under Plan

The following summarizes option activity under the 2018 Plan:


	Outstanding Options
	Number of Options	Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term	Aggregate Intrinsic Value
				(in years)	(in thousands)
Balance, December 31, 2017	—	$	—		$	—
Options granted	12,336,975	5.47
Options exercised	(5,020,580)	2.27			123,808
Options forfeited	(80,850)	2.27
Balance, December 31, 2018	7,235,545	$	7.72	9.62	$	139,001
Options granted	3,052,816	27.47
Options exercised	(711,123)	4.16			$	17,141
Options forfeited	(386,716)	8.79
Balance, December 31, 2019	9,190,522	$	14.51	8.82	$	110,490
Exercisable, December 31, 2019	4,660,416	$	11.33	8.68	$	69,379
Vested and expected to vest, December 31, 2019	9,190,522	$	14.51	8.82	$	110,490

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The fair value of employee, consultant and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2019	2018
Fair value of common stock	$25.94 - $31.99	$2.27 - $26.52
Expected term in years	5.27 - 6.08	5.99 - 6.25
Expected volatility	74.14% - 74.92%	74.2% - 77.0%
Expected risk-free interest rate	1.54% - 2.62%	2.74% - 2.99%
Expected dividend	0%	0%

Schedule of Unvested Restricted Stock Units Activity

The following summarizes restricted stock unit activity under the 2018 Plan:


	Outstanding Restricted Stock Units
	Restricted Stock Units	Weighted- Average Grant Date Fair Value per Share		Weighted Average Remaining Vesting Life	Aggregate Intrinsic Value
				(in years)	(in thousands)
Unvested December 31, 2018	—	—			—
Granted	1,982,855	$	27.45	1.94
Vested	(500)	25.94
Forfeited	(41,200)	27.48
Unvested December 31, 2019	1,941,155	$	27.45	1.98	$	50,431
Vested and expected to vest, December 31, 2019	1,941,155	$	27.45	1.98	$	50,431

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

The fair values of the rights granted under the ESPP were calculated using the following assumptions:


	Year ended December 31,
	2019	2018
Expected term (in years)	0.50 – 2.00	0.50 – 2.00
Volatility	60.4% - 76.0%	67.7% - 81.8%
Risk-free interest rate	1.72%-2.49%	2.37% - 2.83%
Dividend yield	—	—

Schedule of Stock-Based Compensation Expense

Total stock-based compensation expense related to stock options, employee stock purchase plans and vesting of the founders’ common stock was as follows:


	Year Ended December 31,
	2019		2018
	(in thousands)
Research and development	$	19,429	$	1,657
General and administrative	26,634		16,909
Total stock-based compensation expense	$	46,063	$	18,566

Cover Page - USD ($) $ in Millions	12 Months Ended
Cover Page - USD ($) $ in Millions	Dec. 31, 2019	Feb. 24, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-38693
Entity Registrant Name	Allogene Therapeutics, Inc.
Entity Central Index Key	0001737287
Current Fiscal Year End Date	--12-31
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	82-3562771
Entity Address, Address Line One	210 East Grand Avenue
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	457-2700
Title of 12(b) Security	Common Stock, Par Value $0.001 Per Share
Trading Symbol	ALLO
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 1,601
Entity Common Stock, Shares Outstanding		124,906,945
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the 2020 Annual Meeting of Stockholders, which will be filed with the Securities and Exchange Commission within 120 days after the end of the Registrant’s fiscal year ended December 31, 2019, are incorporated by reference into Part III of this Report.

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Prepayments and Other Current Assets

Prepaid Expenses and Other Current Assets


	December 31,
	2019		2018
	(in thousands)
Prepaid research and development expenses	$	6,387	$	2,356
Prepaid insurance	2,625		2,376
Accrued interest on short-term marketable securities	2,403		3,108
Other prepaid and current assets	2,628		758
Total prepaid expenses and other current assets	$	14,043	$	8,598

Schedule of Property and Equipment, Net

Property and Equipment, Net


	December 31,
	2019		2018
	(in thousands)
Leasehold improvements	$	29,924	$	15
Laboratory equipment	13,117		5,534
Construction in progress	12,390		2,703
Computers equipment and purchased software	3,726		1,327
Furniture and fixtures	2,764		64
Total	61,921		9,643
Less: accumulated depreciation	(5,472)		(1,048)
Total property and equipment, net	$	56,449	$	8,595

Schedule of Intangible Assets, Net

Intangible Assets, Net


	December 31, 2019
	Cost		Accumulated Amortization		Carrying Value
	(in thousands)
Assembled workforce	$	1,206	$	(1,055)	$	151


	December 31, 2018
	Cost		Accumulated Amortization		Carrying Value
	(in thousands)
Assembled workforce	$	1,206	$	(452)	$	754

Schedule of Accrued Liabilities

Accrued Liabilities

Accrued liabilities consist of the following:


	December 31,
	2019		2018
	(in thousands)
Accrued compensation and related benefits	$	9,560	$	4,111
Accrued research and development expenses	4,833		7,808
Accrued property and equipment	3,575		—
Unvested shares liabilities	2,843		4,590
Other	3,018		612
Total accrued and other current liabilities	$	23,829	$	17,121

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

2018 Equity Incentive Plan

In June 2018, the Company adopted the 2018 Equity Incentive Plan (2018 Plan). The 2018 Plan provided for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the Company’s Board of Directors and consultants of the Company under terms and provisions established by the Company’s Board of Directors. In October 2018, the Board of Directors approved an amendment and restatement of the 2018 Plan, increasing the shares of common stock issuable under the 2018 Plan as well as allowing for an automatic annual increase to the shares issuance under the 2018 Plan to the amount equal to 5% of the total number of shares of common stock outstanding on December 31st of the preceding calendar year. The term of any stock option granted under the 2018 Plan cannot exceed 10 years. The Company generally grants stock-based awards with service conditions only. Options granted typically vest over a four-year period but may be granted with different vesting terms. Restricted Stock Units granted typically vest annually over a four-year period but may be granted with different vesting terms. Options shall not have an exercise price less than 100% of the fair market value of the Company’s common stock on the grant date. If the individual possesses more than 10% of the combined voting power of all classes of stock of the Company, the exercise price shall not be less than 110% of the fair market value of a common share of stock on the date of grant. This requirement is applicable to incentive stock options only.

As of December 31, 2019 and 2018, there were 9,642,503 and 8,176,125 shares reserved by the Company under the 2018 Plan for the future issuance of equity awards.

Stock Option Activity

The following summarizes option activity under the 2018 Plan:


	Outstanding Options
	Number of Options	Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term	Aggregate Intrinsic Value
				(in years)	(in thousands)
Balance, December 31, 2017	—	$	—		$	—
Options granted	12,336,975	5.47
Options exercised	(5,020,580)	2.27			123,808
Options forfeited	(80,850)	2.27
Balance, December 31, 2018	7,235,545	$	7.72	9.62	$	139,001
Options granted	3,052,816	27.47
Options exercised	(711,123)	4.16			$	17,141
Options forfeited	(386,716)	8.79
Balance, December 31, 2019	9,190,522	$	14.51	8.82	$	110,490
Exercisable, December 31, 2019	4,660,416	$	11.33	8.68	$	69,379
Vested and expected to vest, December 31, 2019	9,190,522	$	14.51	8.82	$	110,490

The aggregate intrinsic values of options exercised, outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock on the Nasdaq Global Select Market on December 31, 2019. During the years ended December 31, 2019 and 2018, the estimated weighted-average grant-date fair value of employee options granted was $18.42 per share and $3.75 per share, respectively. As of December 31, 2019 and 2018, there was $74.7 million and $42.8 million, respectively, of unrecognized stock-based compensation related to unvested stock options, which is expected to be recognized over a weighted-average period of 3 years, 15 days and 3 years, 6 months, respectively.

The fair value of employee, consultant and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2019	2018
Fair value of common stock	$25.94 - $31.99	$2.27 - $26.52
Expected term in years	5.27 - 6.08	5.99 - 6.25
Expected volatility	74.14% - 74.92%	74.2% - 77.0%
Expected risk-free interest rate	1.54% - 2.62%	2.74% - 2.99%
Expected dividend	0%	0%

The Black-Scholes option-pricing model requires the use of subjective assumptions which determine the fair value of stock-based awards. These assumptions include:

Fair value of common stock—For grants before October 2018 when the Company was private and there was no public market for the Company’s common stock, the fair value of the Company’s common stock underlying share-based awards was estimated on each grant date by the Company’s board of directors. In order to determine the fair value of the Company’s common stock underlying option grants, the Company’s board of directors considered, among other things, valuations of the Company’s common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. For all grants subsequent to the Company’s IPO in October 2018, the fair value of common stock was determined by taking the closing price per share of common stock per Nasdaq.

Expected term— The expected term represents the period that stock-based awards are expected to be outstanding. The expected term for option grants is determined using the simplified method. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the stock-based awards.

Expected volatility— The Company uses an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have sufficient trading history for its common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

Risk-free interest rate—The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option.

Expected dividend—The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.

For the years ended December 31, 2019 and 2018, total stock-based compensation expense related to stock options was $21.9 million and $3.3 million, respectively.

Restricted Stock Unit Activity

The following summarizes restricted stock unit activity under the 2018 Plan:


	Outstanding Restricted Stock Units
	Restricted Stock Units	Weighted- Average Grant Date Fair Value per Share		Weighted Average Remaining Vesting Life	Aggregate Intrinsic Value
				(in years)	(in thousands)
Unvested December 31, 2018	—	—			—
Granted	1,982,855	$	27.45	1.94
Vested	(500)	25.94
Forfeited	(41,200)	27.48
Unvested December 31, 2019	1,941,155	$	27.45	1.98	$	50,431
Vested and expected to vest, December 31, 2019	1,941,155	$	27.45	1.98	$	50,431

  In September 2019, the Company granted 57,361 performance based restricted stock units to a certain executive officer pursuant to the 2018 Plan. These performance awards are subject to the holder's continued service to the Company through each applicable vesting event. Through December 31, 2019, the Company believes that the achievement of the requisite performance conditions for these awards are not probable and as a result, no compensation expense has been recognized related to these awards in the year ended December 31, 2019.

For the years ended December 31, 2019 and 2018, total stock-based compensation expense related to restricted stock units was $8.8 million and zero, respectively. As of December 31, 2019 and 2018, there was $43.0 million and zero, respectively, of unrecognized stock-based compensation which is expected to be recognized over a weighted average period of 1.98 years.

Employee Stock Purchase Plan

In October 2018, the shareholders approved the 2018 Employee Stock Purchase Plan (ESPP), which initially reserved 1,160,000 shares of our common stock for employee purchases under terms and provisions established by the Board of Directors. Effective January 1, 2019, the number of shares authorized under the ESPP for employee purchases increased by 1,214,826 shares. The ESPP is intended to qualify as an ‘employee stock purchase plan’ under Section 423 of the Internal Revenue Code. Under the current offering adopted pursuant to the ESPP, each offering period is approximately 24 months, which is generally divided into four purchase periods of approximately six months.

Employees are eligible to participate if they are employed by the Company. Under the ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the purchase date. The ESPP provides for consecutive, overlapping 24-month offering periods. The offering periods are scheduled to start on the first trading day on or after March 16 or September 16 of each year, except for the first offering period which commenced on October 11, 2018, the first trading day after the effective date of the Company’s registration statement. Contributions under the ESPP are limited to a maximum of 15% of an employee’s eligible compensation.

The fair values of the rights granted under the ESPP were calculated using the following assumptions:


	Year ended December 31,
	2019	2018
Expected term (in years)	0.50 – 2.00	0.50 – 2.00
Volatility	60.4% - 76.0%	67.7% - 81.8%
Risk-free interest rate	1.72%-2.49%	2.37% - 2.83%
Dividend yield	—	—

For the years ended December 31, 2019 and 2018, total stock-based compensation expense related to ESPP was $1.6 million and $0.4 million, respectively.

Founders’ Stock

Stock-based compensation expense is recognized for shares of founders’ stock as vesting conditions are met. In relation to the modification described in Note 12, 24,230,750 shares of founders’ stock remained unvested at the modification date in April 2018. For the years ended December 31, 2019 and 2018, $13.7 million and $14.9 million of stock-based compensation expense was recognized related to the vesting of 6,057,684 and 6,562,506 shares, respectively, of founders' stock. At December 31, 2019 and 2018, there was $30.9 million and $44.6 million of unrecognized stock-based compensation expense related to 13,629,803 and 19,687,487 shares of unvested founders’ stock which is expected to be recognized over 2 years, 3 months and 3 years, 3 months, respectively. The weighted-average fair value at grant date for founders’ stock was $2.27 per share.

Total stock-based compensation expense related to stock options, employee stock purchase plans and vesting of the founders’ common stock was as follows:


	Year Ended December 31,
	2019		2018
	(in thousands)
Research and development	$	19,429	$	1,657
General and administrative	26,634		16,909
Total stock-based compensation expense	$	46,063	$	18,566

Early Exercised Options

The Company allows certain of its employees and its directors to exercise options granted under the 2018 Plan prior to vesting. The shares related to early exercised stock options are subject to the Company’s lapsing repurchase right upon termination of employment or service on the Company’s Board of Directors at the lesser of the original purchase price or fair market value at the time of repurchase. In order to vest, the holders are required to provide continued service to the Company. The proceeds are initially recorded in accrued and other liabilities and other long-term liabilities for the noncurrent portion. The proceeds are reclassified to paid-in capital as the repurchase right lapses. During the years ended December 31, 2019 and 2018, zero and 5,020,580 options were early exercised. As of December 31, 2019 and 2018, there was $2.8 million and $4.6 million recorded in accrued and other liabilities and $3.9 million and $6.8 million recorded in other long-term liabilities related to shares held by employees and directors that were subject to repurchase. The underlying shares are shown as outstanding in the financial statements since the exercise date.

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	124,267,358	121,482,671
Common stock, shares outstanding (in shares)	124,267,358	121,482,671

Statements of Cash Flows - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (2)	$ (184,594)	$ (211,505)
Adjustments to reconcile net loss to net cash (used in) operating activities:
Acquired in-process research and development	0	0	109,436
Stock-based compensation	0	46,063	18,566
Amortization of other intangible assets acquired	0	603	452
Depreciation and amortization	0	4,424	1,048
Net amortization/accretion on investment securities	0	(3,596)	(1,036)
Non-cash rent expense	0	6,777	1,832
Change in fair value of convertible notes payable	0	0	21,211
Debt issuance costs on convertible notes payable	0	0	3,358
Income tax benefit	0	(331)	(117)
Share of losses from equity method investments	0	182	0
Other	0	0	6
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	0	(5,445)	(8,598)
Other long-term assets	0	(4,374)	(244)
Accounts payable	0	(985)	8,800
Accrued and other current liabilities	2	6,351	12,138
Other long-term liabilities	0	(2,425)	0
Net cash used in operating activities	0	(137,350)	(44,653)
Cash flows from investing activities:
Purchases of property and equipment	0	(50,791)	(3,234)
Purchase of stock in equity method investment	0	(5,075)	0
Proceeds from sales of investments	0	0	2,606
Proceeds from maturities of investments	0	472,578	19,235
Purchase of investments	0	(252,628)	(649,307)
Cash paid for acquisition of assets	0	0	(2,098)
Net cash provided by (used in) investing activities	0	164,084	(632,798)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	299,281
Proceeds from issuance of convertible notes, net of issuance costs	0	0	116,842
Proceeds from early exercise of stock options	0	0	11,370
Proceeds from issuance of common stock, net of commissions and issuance costs	0	54,219	343,689
Proceeds from issuance of common stock and upon exercise of stock options	0	2,958	0
Proceeds from issuance of common stock under the employee stock purchase plan	0	1,783	0
Net cash provided by financing activities	0	58,960	771,182
Net increase in cash, cash equivalents and restricted cash	0	85,694	93,731
Cash, cash equivalents and restricted cash — beginning of period	0	93,731	0
Cash, cash equivalents and restricted cash — end of period	0	179,425	93,731
Non-cash operating, investing and financing activities:
Common stock issued on conversion of convertible preferred stock	0	0	411,052
Common stock issued on conversion of convertible notes payable	0	0	141,410
Series A-1 convertible preferred stock issued in asset acquisition	0	0	111,770
PP&E and other assets acquired in asset acquisition	0	0	111,770
Right-of-use asset obtained in exchange for lease liability	0	13,827	33,015
Property and equipment purchases in accounts payable and accrued liabilities	0	4,668	3,182
Deferred offering costs included in accounts payable and accrued and other current liabilities	0	135	356
Supplemental disclosure:
Cash paid for amounts included in the measurement of lease liabilities	0	(3,563)	(31)
Cash received for amounts related to tenant improvement allowances from lessors	$ 0	$ 4,473	$ 0

Description of Business and Summary of Significant Accounting Policies - Additional Information (Details)		1 Months Ended			3 Months Ended								12 Months Ended		25 Months Ended
	Oct. 01, 2018	Nov. 30, 2019 USD ($)	Oct. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2019 USD ($) shares	Oct. 10, 2018 shares
Subsidiary, Sale of Stock [Line Items]
Start up costs				$ 2,000,000
Issuance of common stock (in shares) \| shares													1,965,082
Offering costs													$ 1,600,000
Preferred stock outstanding (in shares) \| shares					0				0				0	0	0
Stock issuance, maximum value		$ 250,000,000.0
Stock issuance, maximum compensation due to third party (as a percentage of stock sales)					3.00%								3.00%		3.00%
Stock issued, value				5,000									$ 54,219,000
Deferred offering costs			$ 3,200,000		$ 0				$ 0				0	$ 0	$ 0
Deferred offering costs reclassified to APIC		$ 500,000
Common stock forward split ratio	0.1905
Cash and cash equivalents and marketable securities					588,900,000								588,900,000		588,900,000
Net loss and comprehensive loss				$ (2,000)	$ (61,030,000)	$ (50,735,000)	$ (41,243,000)	$ (31,586,000)	$ (30,509,000)	$ (43,497,000)	$ (134,902,000)	$ (2,597,000)	(184,594,000)	(211,505,000)	$ (396,100,000)
Disposal of property, plant, and equipment													$ 200,000	0
Acquired in process research and development														109,400,000
IPO
Subsidiary, Sale of Stock [Line Items]
Issuance of common stock (in shares) \| shares			20,700,000
Common stock price (in dollars per share) \| $ / shares			$ 18.00
Net proceeds			$ 343,300,000
Underwriting discounts and commissions			26,100,000
Offering costs			$ 3,200,000											29,272,000
Number of 2018 Notes converted into common stock (in shares) \| shares			7,856,176
Preferred stock outstanding (in shares) \| shares			0
Stock issued, value														$ 343,329,000
Convertible preferred stock, shares outstanding (in shares) \| shares			11,743,987
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Issuance of common stock (in shares) \| shares			2,700,000
Common Stock
Subsidiary, Sale of Stock [Line Items]
Issuance of common stock (in shares) \| shares				26,249,993									1,965,082
Number of 2018 Notes converted into common stock (in shares) \| shares			7,856,176
Stock issued, value				$ 26,000									$ 2,000
Common Stock \| IPO
Subsidiary, Sale of Stock [Line Items]
Issuance of common stock (in shares) \| shares														20,700,000
Number of convertible preferred stock converted to shares of common stock (in shares) \| shares			61,655,922													61,655,922
Number of 2018 Notes converted into common stock (in shares) \| shares														7,856,176
Stock issued, value														$ 21,000

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventsFrom January 6 to January 17, 2020, the Company sold an aggregate of 570,839 shares of common stock in ATM offerings resulting in net proceeds of $14.8 million.

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Summary of Undiscounted Future Non-Cancellable Lease Payments Under Operating Leases

The undiscounted future non-cancellable lease payments under our operating leases as of December 31, 2019 is as follows:


Year ending December 31:	(in thousands)
2020	$	5,662
2021	7,632
2022	7,951
2023	8,203
2024 and thereafter	63,864
Total undiscounted lease payments	93,312
Less: Present value adjustment	(35,920)
Less: Tenant improvement allowance	(4,367)
Total	$	53,025

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Purchase Commitments In the normal course of business, the Company enters into various purchase commitments with third-party contract manufacturers for the manufacture and processing of our product candidates and related raw materials, and we have entered into other contracts in the normal course of business with contract research organizations for clinical trials and other vendors for other services and products for operating purposes. These agreements generally provide for termination or cancellation, other than for costs already incurred. Contingencies In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnifications. The Company’s exposure under these agreements is unknown, because it involves claims that may be made against the Company in the future, but have not yet been made. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. Indemnification In accordance with the Company’s amended and restated certificate of incorporation and amended and restated bylaws, the Company has indemnification obligations to its officers and directors for certain events or occurrences, subject to certain limits, while they are serving in such capacity. There have been no claims to date, and the Company has a directors and officers liability insurance policy that may enable it to recover a portion of any amounts paid for future claims.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2019

Selected Quarterly Financial Information [Abstract]

Selected Quarterly Financial Data (unaudited)

The following table provides the selected quarterly financial data for the year ended December 31, 2019 (in thousands, except per share amounts):


	Quarter Ended
	March 31, 2019		June 30, 2019		September 30, 2019		December 31, 2019
Loss from operations	$	(36,461)	$	(45,961)	$	(55,011)	$	(64,575)
Net loss	(31,586)		(41,243)		(50,735)		(61,030)
Net loss per share, basic and diluted	$	(0.32)	$	(0.41)	$	(0.50)	$	(0.58)

The following table provides the selected quarterly financial data for the year ended December 31, 2018 (in thousands, except per share amounts):


	Quarter Ended
	March 31, 2018		June 30, 2018		September 30, 2018		December 31, 2018
Loss from operations	$	2,597	$	135,012	$	22,187	$	33,046
Net loss	(2,597)		(134,902)		(43,497)		(30,509)
Net loss per share, basic and diluted	$	(0.10)	$	(43.82)	$	(10.71)	$	(0.37)

Equity Method Investment	12 Months Ended
Equity Method Investment	Dec. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investment	Equity Method Investment In conjunction with the execution of the Notch Agreement (see Note 7), the Company also entered into a Share Purchase Agreement (“Notch Investment Agreement”) with the Company acquiring shares of Notch’s Series Seed convertible preferred stock for a total investment cost of $5.1 million which includes transaction costs of $0.1 million, resulting in a 25% ownership interest in Notch. The Company’s total equity investment in Notch as of December 31, 2019 was $4.9 million and the Company accounted for the investment using the cost method of accounting. During the year ended December 31, 2019, the Company recognized its share of Notch's net loss under the other expenses caption within the statement of operations. The Company's share of Notch's net loss was $0.2 million for the year ended December 31, 2019.

401(k) Plan	12 Months Ended
401(k) Plan	Dec. 31, 2019
Retirement Benefits [Abstract]
401(k) Plan	401(k) PlanIn April 2018, the Company began to sponsor a 401(k) retirement savings plan for the benefit of its employees. All employees are eligible to participate, provided they meet the requirements of the plan. The Company made contributions to the plan for eligible participants, and recorded contribution expenses of $0.9 million and $0.4 million related to matched contributions for the year ended December 31, 2019 and 2018, respectively.

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 0	$ 144,535	$ 151,860
General and administrative	2	57,473	40,982
Total operating expenses	2	202,008	192,842
Loss from operations	(2)	(202,008)	(192,842)
Other income (expense), net:
Change in fair value of convertible note payable	0	0	(21,211)
Interest expense	0	0	(3,358)
Interest and other income, net	0	17,351	5,789
Other expenses	0	(268)	0
Total other income (expense), net	0	17,083	(18,780)
Loss before income taxes	(2)	(184,925)	(211,622)
Benefit from income taxes	0	331	117
Net loss	(2)	(184,594)	(211,505)
Other comprehensive income:
Net unrealized gain on available-for-sale investments, net of tax	0	839	306
Net comprehensive loss	$ (2)	$ (183,755)	$ (211,199)
Net loss per share, basic and diluted (in dollars per share)	$ 0.00	$ (1.83)	$ (7.31)
Weighted-average number of shares used in computing net loss per share, basic and diluted (in shares)	26,249,993	101,061,149	28,948,386

Convertible Preferred Stock and Stockholders' Deficit (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Apr. 30, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2018	Oct. 10, 2018	Aug. 31, 2018	Nov. 29, 2017
Class of Stock [Line Items]
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 0	$ 0	$ 299,281
Aggregate purchase price, shares value	$ 300,000
Subscriptions receivable							$ 150,000
Preferred stock, shares authorized (in shares)			10,000,000	10,000,000
Preferred stock, shares issued (in shares)			0	0
Preferred stock, shares outstanding (in shares)			0	0
Common stock, shares authorized (in shares)			200,000,000	200,000,000
Common stock, shares issued (in shares)			124,267,358	121,482,671
Common stock, shares outstanding (in shares)			124,267,358	121,482,671
Modified shares of common stock (in shares)			26,249,993
Compensation cost resulting from the modification			$ 59,500
Vesting term			4 years
Dividends declared on common stock			$ 0	$ 0
IPO
Class of Stock [Line Items]
Common stock price (in dollars per share)					$ 18.00
Preferred stock, shares outstanding (in shares)					0
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)		26,249,993	124,267,358	121,482,671				0
Common Stock \| IPO
Class of Stock [Line Items]
Number of convertible preferred stock converted to shares of common stock (in shares)					61,655,922	61,655,922
Series A1 Convertible Preferred Shares
Class of Stock [Line Items]
Convertible preferred stock, shares issued (in shares)	3,187,772
Convertible preferred stock, shares issued (in shares)	998,225
Common stock price (in dollars per share)	$ 35.06
Proceeds from issuance of convertible preferred stock, net of issuance costs	$ 34,900
Series A Convertible Preferred Shares
Class of Stock [Line Items]
Convertible preferred stock, shares issued (in shares)	7,557,990
Common stock price (in dollars per share)	$ 35.06
Proceeds from issuance of convertible preferred stock, net of issuance costs	$ 264,400

Subsequent Events (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
Subsequent Events (Details) - USD ($) $ in Thousands	Jan. 17, 2020	Dec. 31, 2017	Dec. 31, 2019
Subsequent Event [Line Items]
Issuance of common stock (in shares)			1,965,082
Stock issued, value		$ 5	$ 54,219
Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock (in shares)	570,839
Stock issued, value	$ 14,800

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 54,018	$ 16,437	$ 0
Tax credit carryforwards	4,239	1,239	0
Intangibles	22,770	23,086	0
Accrued expenses	2,375	952	0
Lease liabilities	14,839	9,730	0
Stock based compensation	6,870	360	0
Other	0	0	0
Total deferred tax assets	105,111	51,804	0
Deferred tax liabilities:
Fixed assets	(361)	(531)	0
Right of use leased assets	(12,453)	(9,239)	0
Investments	(393)	(118)	0
Other	0	0	0
Total deferred tax liabilities	(13,207)	(9,888)	0
Net deferred tax assets	91,904	41,916	0
Valuation allowance	(91,904)	(41,916)	0
Net deferred tax assets	$ 0	$ 0	$ 0

Convertible Preferred Stock and Stockholders' Equity	12 Months Ended
Convertible Preferred Stock and Stockholders' Equity	Dec. 31, 2019
Equity [Abstract]
Convertible Preferred Stock and Stockholders’ Equity	Convertible Preferred Stock and Stockholders’ Equity Convertible Preferred Stock As discussed in Note 6, the Company issued 3,187,772 shares of its Series A-1 convertible preferred stock to Pfizer in connection with the Pfizer Agreement entered into in April 2018. In April 2018, the Company issued 7,557,990 shares of its Series A convertible preferred stock at a price per share of $35.06 for net cash proceeds of $264.4 million and issued 998,225 shares of Series A-1 convertible preferred stock at a price per share of $35.06 for net cash proceeds of $34.9 million. Fifty percent of the aggregate purchase price of $300.0 million was paid in April 2018. The remaining subscriptions receivable of $150.0 million was received in July and August 2018, at the election of the Company’s board of directors. On the completion of the IPO (see Note 1), all outstanding shares of convertible preferred stock were automatically converted into 61,655,922 shares of common stock. Preferred Stock Pursuant to the Amended and Restated Certificate of Incorporation filed on October 15, 2018, as amended, the Company is authorized to issue a total of 10,000,000 shares of preferred stock, of which no shares were issued and outstanding at December 31, 2019 and 2018. Common Stock Pursuant to the Amended and Restated Certificate of Incorporation filed on October 15, 2018, as amended, the Company is authorized to issue a total of 200,000,000 shares of common stock, of which 124,267,358 and 121,482,671 shares were issued and outstanding at December 31, 2019 and 2018, respectively. In connection with the issuance of the Company’s Series A convertible preferred stock in April 2018, the Company’s founders agreed to modify their common shares outstanding to include vesting provisions that require continued service to the Company in order to vest in those shares. As such, the 26,249,993 modified shares of common stock became compensatory upon such modification. The total compensation cost resulting from the modification is approximately $59.5 million and is being recognized over the four year vesting term. Common stockholders are entitled to dividends if and when declared by the Company’s Board of Directors subject to the prior rights of the preferred stockholders. As of December 31, 2019 and 2018, no dividends on common stock had been declared by the Company’s board of directors.

Stock-Based Compensation - Summary of Stock Option Activity Under Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, vested and expected to vest	$ 110,490
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options, beginning balance (in shares)	7,235,545	0
Number of options, options granted	3,052,816	12,336,975
Number of options, options exercised (in shares)	(711,123)	(5,020,580)
Number of options, options forfeited (in shares)	(386,716)	(80,850)
Number of options, ending balance (in shares)	9,190,522	7,235,545	0
Number of options, exercisable (in shares)	4,660,416
Number of options, vested and expected to vest (in shares)	9,190,522
Weighted-average exercise price, beginning balance (in dollars per share)	$ 7.72	$ 0
Weighted-average exercise price, options granted (in dollars per share)	27.47	5.47
Weighted-average exercise price, options exercised (in dollars per share)	4.16	2.27
Weighted-average exercise price, options forfeited (in dollars per share)	8.79	2.27
Weighted-average exercise price, ending balance (in dollars per share)	14.51	$ 7.72	$ 0
Weighted-average exercise price, exercisable (in dollars per share)	11.33
Weighted-average exercise price, vested and expected to vest (in dollars per share)	$ 14.51
Weighted-average remaining contract term	8 years 9 months 25 days
Weighted-average remaining contract term, exercisable	8 years 8 months 4 days
Weighted-average remaining contract term, vested and expected to vest	8 years 9 months 25 days	9 years 7 months 13 days
Aggregate intrinsic value, exercised	$ 17,141	$ 123,808
Aggregate intrinsic value, balance	110,490	$ 139,001
Aggregate intrinsic value, exercisable	$ 69,379
Aggregate intrinsic value, vested and expected to vest			$ 0

Description of Business and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Text Block [Abstract]

Estimated Life Of Assets

The Company has determined the estimated life of assets to be as follows:


Laboratory equipment			5 years
Computer equipment and purchased software			3 - 5 years
Fixtures and furniture			7 years
Leasehold improvements			Shorter of lease term or useful life

Recent Accounting Guidance	12 Months Ended
Recent Accounting Guidance	Dec. 31, 2019
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Guidance	Recent Accounting Guidance Recently Adopted Accounting Pronouncements In January 2017, the FASB issued Accounting Standards Update, Business Combinations (Topic 805): Clarifying the Definition of a Business (ASU 2017-01). ASU 2017-01 clarifies the framework for determining whether an integrated set of assets and activities meets the definition of a business. The revised framework establishes a screen for determining whether an integrated set of assets and activities is a business and narrows the definition of a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. This screen reduces the number of transactions that need to be further evaluated. This new accounting guidance is effective for public or private companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The new accounting guidance should be applied prospectively on or after the effective date. The Company adopted this guidance on January 1, 2018. In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (ASU 2018-07). ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Some of the areas of simplification apply only to nonpublic entities. For all entities, the amendments are effective for annual periods beginning after December 15, 2019, and interim periods within annual periods beginning after December 15, 2020. Early adoption is permitted for any entity in any interim or annual period for which financial statements have not been issued or made available for issuance, but not before an entity adopts ASC 606. The Company early adopted this guidance on January 1, 2018. As a result, the accounting for share-based payments to nonemployee consultants is consistent with employees. In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows: Restricted Cash. This ASU requires changes in restricted cash during the period to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. If cash, cash equivalents and restricted cash are presented in more than one line item on the balance sheet, the new guidance requires a reconciliation of the total in the statement of cash flows to the related captions in the balance sheet. This guidance is effective for annual and interim periods of public entities beginning after December 15, 2017, with early adoption permitted. The amendments in this ASU should be applied retrospectively to all periods presented. The Company adopted this guidance on January 1, 2018. The adoption of this ASU increased our ending cash balances within the statements of cash flows. The adoption had no other material impacts to the statements of cash flows and had no impact on the results of operations or financial position. In February 2016, the FASB issued Accounting Standards Update No. 2016-02, Leases (ASU 2016-02), which provides revised accounting requirements for both lessees and lessors. Lessees will recognize a right-of-use asset and a lease liability for virtually all leases (other than short-term leases upon election). The liability is recognized at the present value of future lease payments. The asset is recognized based on the liability. For statements of operations purposes, ASU 2016-02 requires leases to be classified as either operating or finance. Operating leases will result in straight-line expense while finance leases will result in a front-loaded expense pattern. ASU 2016-02 is effective for public companies for fiscal years beginning after December 15, 2018. Early adoption is permitted. The standard requires a modified-retrospective transition method and provides for certain practical expedients. The Company early adopted the new lease standard on July 1, 2018 with the adoption reflected as of January 1, 2018 in accordance with ASU No. 2018-11, Leases (Topic 842) –Targeted Improvements. There were no lease arrangements prior to August 2018 and consequently, the adoption of the standard did not have any impact on periods prior to August 2018. In February 2018, the FASB issued Accounting Standards Update No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which provided amended guidance to allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. Additionally, under the new guidance, an entity will be required to provide certain disclosures regarding stranded tax effects. The guidance is effective for fiscal years beginning after December 15, 2018, with early adoption permitted. The Company adopted this guidance on January 1, 2019. Adoption of the new guidance had no significant impact on the Company’s financial statements. Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments and also issued subsequent amendments to the initial guidance: ASU 2018-19, ASU 2019-04, ASU 2019-05, and ASU 2019-11. The standard requires measurement and recognition of expected credit losses for financial assets by requiring an allowance to be recorded as an offset to the amortized cost of such assets. For available-for-sale debt securities, expected credit losses should be estimated when the fair value of the debt securities is below their associated amortized costs. The standard will become effective for the Company in the first quarter of 2020, with early adoption permitted beginning the first quarter of 2019. The modified retrospective approach should be applied upon adoption of this new guidance. The Company’s financial instruments that are in the scope of ASU 2016-13 include but not limited to trade receivables and available-for-sale debt securities. The Company will adopt this standard on January 1, 2020 and does not anticipate this amendment to have a material impact on the financial statements. In August 2018, the FASB issued Accounting Standards Update No. 2018-15, Intangibles – Goodwill and other – Internal-Use Software (Subtopic 350-40), which amended its guidance for costs of implementing a cloud computing service arrangement and aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. The guidance is effective for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company is currently evaluating the impact of adopting this amendment on the financial statements. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which clarifies that certain transactions between participants in a collaborative arrangement should be accounted for under ASC 606 when the counterparty is a customer. In addition, Topic 808 precludes an entity from presenting consideration from a transaction in a collaborative arrangement as revenue from contracts with customers if the counterparty is not a customer for that transaction. The Company will adopt this standard on January 1, 2020 and does not anticipate this amendment to have a material impact on the financial statements. In December 2019, the FASB issued Accounting Standard Update No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (ASU 2019-12), which simplifies the accounting for income taxes, eliminates certain exceptions within ASC 740, Income Taxes, and clarifies certain aspects of the current guidance to promote consistency among reporting entities. This guidance will be effective for the Company in the first quarter of 2021 on a prospective basis, and early adoption is permitted. The Company is currently evaluating the impact of the new guidance on the financial statements. In January 2020, the FASB issues Accounting Standard Update No. 2020-01, Investments – Equity Securities (Topic 321), Investments – Equity Method and Joint Ventures (Topic 323), which clarifies the interactions between topics 321 and 323 in applying or discontinuing the equity method of accounting for investments. This guidance will be effective for the Company in the first quarter of 2021, and early adoption is permitted. The Company is currently evaluating the impact of the new guidance on the financial statements.

Investments

12 Months Ended

Dec. 31, 2019

Investments, All Other Investments [Abstract]

Investments

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2019 are presented in the following table:


	December 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
	(in thousands)
Money market funds	$	122,900	$	—	$	—	$	122,900
Corporate bonds	204,144		871		(4)		205,011
U.S. treasury securities	181,340		557		(3)		181,894
U.S. agency securities	25,658		167		(1)		25,824
Certificates of deposit	1,000		—		—		1,000
Total cash equivalents and investments	$	535,042	$	1,595	$	(8)	$	536,629

Classified as:
Cash equivalents							$	122,900
Short-term investments							355,407
Long-term investments							58,322
Total cash equivalents, and investments							$	536,629

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2018 are presented in the following table:


	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
	(in thousands)
Money market funds	$	61,023	$	—	$	—	$	61,023
Commercial Paper	4,917		—		—		4,917
Corporate bonds	244,136		220		(280)		244,076
U.S. treasury securities	341,696		342		(37)		342,001
U.S. agency securities	61,937		181		(3)		62,115
Total cash equivalents and investments	$	713,709	$	743	$	(320)	$	714,132

Classified as:
Cash equivalents							$	85,214
Short-term investments							366,952
Long-term investments							261,966
Total cash equivalents, and investments							$	714,132

The fair values of available-for-sale debt investments by contractual maturity as of December 31, 2019 and 2018 were as follows:


	December 31,
	2019		2018
	(in thousands)
Due in 1 year or less	$	355,407	$	375,625
Due in 1 - 2 years	58,322		240,614
Due in 3 years	—		36,870
Instruments not due at a single maturity date	122,900		61,023
Total cash equivalents and investments	$	536,629	$	714,132

As of December 31, 2019 and 2018, the remaining contractual maturities of available-for-sale securities were less than two years and three years, respectively. There have been no significant realized losses on available-for-sale securities for the years ended December 31, 2019 and 2018. Based on the Company's review of its available-for-sale securities, the Company believes it had no other-than-temporary impairments on these securities as of December 31, 2019 and 2018, because the Company does not intend to sell these securities nor does the Company believe that it will be required to sell these securities before the recovery of their amortized cost basis. Gross realized gains and gross realized losses were immaterial for the years ended December 31, 2019 and 2018.

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Oct. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Share-based compensation expense		$ 46,063,000	$ 18,566,000
Accrued and other liabilities, related to shares held by employees and directors that were subject to repurchase		$ 2,800,000	$ 4,600,000
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares as percentage of common shares outstanding		5.00%
Number of shares reserved for future issuance (in shares)		9,642,503	8,176,125
Issuance of common stock for exercise of stock options (in shares)		711,123	5,020,580
2018 Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power by individual		10.00%
2018 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted period		10 years
Option exercise price as percentage of fair value of common stock on grate date		100.00%
Option exercise price as percentage of fair value of common stock on grate date		110.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Estimated weighted average grant date fair value of employee options granted (in dollars per share)		$ 18.42	$ 3.75
Unrecognized stock based compensation		$ 74,700,000	$ 42,800,000
Weighted-average period recognized		3 years 15 days	3 years 6 months
Period of recognized compensation cost		3 years 15 days	3 years 6 months
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Unrecognized stock based compensation		$ 43,000,000.0	$ 0
Weighted-average period recognized		1 year 11 months 23 days
Share-based compensation expense		$ 8,800,000	$ 0
Period of recognized compensation cost		1 year 11 months 23 days
Restricted stock units, weighted average grant date fair value, grants (in dollars per share)		$ 27.45
Restricted stock units, granted (in shares)		1,982,855
Restricted Stock Units \| Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, granted (in shares)			57,361
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 1,600,000	$ 400,000
Initially reserved common stock for employee purchase (in shares)	1,160,000	1,214,826
Current offering period	24 months
Purchase of common stock through payroll deductions to equal price of lower fair market value	85.00%
Percentage of eligible compensation contribution by employee	15.00%
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 21,900,000	3,300,000
Founders Stock Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation		$ 30,900,000	$ 44,600,000
Weighted-average period recognized		2 years 3 months	3 years 3 months
Number of vested shares (in shares)		6,057,684	6,562,506
Share-based compensation expense		$ 13,700,000	$ 14,900,000
Number of shares unvested (in shares)		13,629,803	19,687,487
Period of recognized compensation cost		2 years 3 months	3 years 3 months
Restricted stock units, weighted average grant date fair value, grants (in dollars per share)		$ 2.27
Founders Stock Award \| Founders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares unvested (in shares)				24,230,750
Early Exercised Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock for exercise of stock options (in shares)		0	5,020,580
Other long term liabilities, related to shares held by employees and directors that were subject to repurchase		$ 3,900,000	$ 6,800,000

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued interest on short-term marketable securities	$ 2,403	$ 3,108
Prepaid Insurance	2,625	2,376
Prepaid Research And Development Expenses Current	6,387	2,356
Other prepaid and current assets	2,628	758
Total prepaid expenses and other current assets	$ 14,043	$ 8,598

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Such Measurements by Major Security Type

Financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type as of December 31, 2019 are presented in the following table:


	December 31, 2019
	Level 1		Level 2		Level 3		Fair Value
	(in thousands)
Financial Assets:
Money market funds ¹	$	122,900	$	—	$	—	$	122,900
Corporate bonds	—		205,011		—		205,011
U.S. treasury securities	181,894		—		—		181,894
U.S. agency securities	—		25,824		—		25,824
Certificates of deposit	—		1,000		—		1,000
Total financial assets	$	304,794	$	231,835	$	—	$	536,629

¹ Included within cash and cash equivalents on the Company’s balance sheet

Financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type as of December 31, 2018 are presented in the following table:


	December 31, 2018
	Level 1		Level 2		Level 3		Fair Value
	(in thousands)
Financial Assets:
Money market funds ¹	$	61,023	$	—	$	—	$	61,023
Commercial paper	—		4,917		—		4,917
Corporate bonds	—		244,076		—		244,076
U.S. treasury securities	342,001		—		—		342,001
U.S. agency securities	—		62,115		—		62,115
Total financial assets	$	403,024	$	311,108	$	—	$	714,132

¹ Included within cash and cash equivalents on the Company’s balance sheet

Convertible Notes Payable (2018 Notes) (Details) - USD ($)	1 Months Ended			12 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Aggregate principal amount		$ 120,200,000
Proceeds from issuance of convertible notes, net of issuance costs		116,800,000	$ 0	$ 0	$ 116,842,000
Fair value of notes	$ 141,400,000	120,200,000
Debt conversion settlement price as a percentage of IPO price per share	85.00%
Change in fair value of convertible notes payable				$ 0	$ 21,200,000
Debt issuance of cost		$ 3,400,000
IPO
Short-term Debt [Line Items]
Number of 2018 Notes converted into common stock (in shares)	7,856,176

Description of Business and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Disclosure Text Block [Abstract]

Deferred Offering Costs

Basis of Presentation

The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP).

Use of Estimates

Concentration of Credit and other Risks and Uncertainties

Segments

Cash, Cash Equivalents and Restricted Cash

Investments

Fair Value Measurement

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Property and Equipment, Net

The Company has determined the estimated life of assets to be as follows:


Laboratory equipment			5 years
Computer equipment and purchased software			3 - 5 years
Fixtures and furniture			7 years
Leasehold improvements			Shorter of lease term or useful life

Leases

Equity Method Investments and Variable Interest Entities

Equity Method Investments

Variable Interest Entities

Accrued Research and Development Costs

become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception.

Income Taxes

Stock-Based Compensation

Net Loss Per Share

Comprehensive Loss

Definite-Lived Intangible Assets

Impairment of Long-Lived Assets

Research and Development Expenses

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares
Convertible Preferred stock per share \| $ / shares	$ 0.001
Series A1 Convertible Preferred Shares
Convertible Preferred stock per share \| $ / shares	$ 35.06
Net issuance cost on equity \| $	$ 84
IPO
Net issuance cost on equity \| $	$ 29,272
Series A Convertible Preferred Stock
Convertible Preferred stock per share \| $ / shares	$ 35.06
Net issuance cost on equity \| $	$ 635

Fair Value Measurements - Additional Information (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	$ 536,629	$ 714,132
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	$ 0	$ 0

Asset Acquisition (Tables)

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Schedule of Fair Value of Assets Acquired

The following table summarizes the fair value of assets acquired (in thousands):


Property and equipment	$	3,258
In-process research and development (IPR&D):
Anti-CD19 CAR T cell therapy	103,936
Anti-BCMA CAR T cell therapy	5,500
Assembled workforce	1,206
Total assets acquired	$	113,900

Investments - Summary of Cash Equivalents, Restricted Cash and Investments, Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 535,042	$ 713,709
Unrealized Gains	1,595	743
Unrealized Losses	(8)	(320)
Fair Value	536,629	714,132
Cash equivalents	122,900	85,214
Short-term investments	355,407	366,952
Long-term investments	58,322	261,966
Total cash equivalents, and investments	536,629	714,132
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	122,900	61,023
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	122,900	61,023
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4,917
Unrealized Gains		0
Unrealized Losses		0
Fair Value		4,917
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	204,144	244,136
Unrealized Gains	871	220
Unrealized Losses	(4)	(280)
Fair Value	205,011	244,076
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	181,340	341,696
Unrealized Gains	557	342
Unrealized Losses	(3)	(37)
Fair Value	181,894	342,001
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,658	61,937
Unrealized Gains	167	181
Unrealized Losses	(1)	(3)
Fair Value	25,824	$ 62,115
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,000
Unrealized Gains	0
Unrealized Losses	0
Fair Value	$ 1,000

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 46,063	$ 18,566
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	19,429	1,657
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 26,634	$ 16,909

Net Loss and Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Schedule of Earnings Per Share Basic and Diluted

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,						Period From November 30, 2017 (Inception) to December 31,
	2019		2018		2017
Numerator:
Net loss	$	(184,594)	$	(211,505)	$	(2)
Denominator:
Weighted average common shares outstanding	101,061,149		28,948,386		26,249,993
Net loss per share, basic and diluted	$	(1.83)	$	(7.31)	$	—

Schedule of Anti-dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share

Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:


	Year Ended December 31,			Period From November 30, 2017 (Inception) to December 31,
	2019	2018	2017
Stock options to purchase common stock	9,190,522	7,235,545	—
Restricted stock units subject to vesting	1,941,155	—	—
Expected shares purchased under Employee Stock Purchase Plan	195,161	144,272	—
Founder shares subject to future vesting	13,629,803	19,687,487	—
Early exercised stock options subject to future vesting	2,992,290	5,020,580	—
Total	27,948,931	32,087,884	—

Asset Acquisition - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Asset Acquisition [Line Items]
Discount rate for valuation	16.50%
Pfizer \| Asset Contribution Agreement
Asset Acquisition [Line Items]
Finite-lived intangible asset, useful life	2 years
Milestone payments, per target (in usd per share)		$ 0	$ 0
Aggregate milestone payments	$ 840,000,000.0
Pfizer \| Asset Contribution Agreement \| United States
Asset Acquisition [Line Items]
Milestone payments, per target (in usd per share)	30,000,000.0
Pfizer \| Asset Contribution Agreement \| European Union
Asset Acquisition [Line Items]
Milestone payments, per target (in usd per share)	$ 60,000,000.0
Series A1 Convertible Preferred Shares
Asset Acquisition [Line Items]
Share issued in connection with asset acquisition (in shares)	3,187,772
Share issued in connection with asset acquisition, fair value	$ 111,800,000
Convertible preferred stock, price per share (in usd per share)	$ 35.06
Costs from issuance of preferred shares	$ 2,100,000
Total asset acquisition	$ 113,900,000

Stock-Based Compensation - Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions (Details) - 2018 Equity Incentive Plan - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	74.14%	74.20%
Expected volatility, maximum	74.92%	77.00%
Expected risk-free interest rate, minimum	1.54%	2.74%
Expected risk-free interest rate, maximum	2.62%	2.99%
Expected dividend	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock	$ 25.94	$ 2.27
Expected term in years	5 years 3 months 7 days	5 years 11 months 26 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock	$ 31.99	$ 26.52
Expected term in years	6 years 29 days	6 years 3 months

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

Fair Value MeasurementsThe Company follows authoritative accounting guidance, which among other things, defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is an exit price, representing the amount that would be received

to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability.

The Company measures and reports its cash equivalents, restricted cash, investments and convertible notes payable at fair value.

Money market funds are measured at fair value on a recurring basis using quoted prices and are classified as Level 1. Investments are measured at fair value based on inputs other than quoted prices that are derived from observable market data and are classified as Level 2 inputs, except for investments in U.S. treasury securities which are classified as Level 1.

There were no Level 3 assets or liabilities at December 31, 2019 or 2018.


	December 31, 2019
	Level 1		Level 2		Level 3		Fair Value
	(in thousands)
Financial Assets:
Money market funds ¹	$	122,900	$	—	$	—	$	122,900
Corporate bonds	—		205,011		—		205,011
U.S. treasury securities	181,894		—		—		181,894
U.S. agency securities	—		25,824		—		25,824
Certificates of deposit	—		1,000		—		1,000
Total financial assets	$	304,794	$	231,835	$	—	$	536,629

¹ Included within cash and cash equivalents on the Company’s balance sheet


	December 31, 2018
	Level 1		Level 2		Level 3		Fair Value
	(in thousands)
Financial Assets:
Money market funds ¹	$	61,023	$	—	$	—	$	61,023
Commercial paper	—		4,917		—		4,917
Corporate bonds	—		244,076		—		244,076
U.S. treasury securities	342,001		—		—		342,001
U.S. agency securities	—		62,115		—		62,115
Total financial assets	$	403,024	$	311,108	$	—	$	714,132

¹ Included within cash and cash equivalents on the Company’s balance sheet

The carrying amounts of accounts payable and accrued liabilities approximate their fair values due to their short-term maturities. The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly.

There were no transfers of assets between the fair value measurement levels during the years ended December 31, 2019 or 2018.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Selected Quarterly Financial Information [Abstract]

Schedule of Quarterly Financial Information

The following table provides the selected quarterly financial data for the year ended December 31, 2019 (in thousands, except per share amounts):


	Quarter Ended
	March 31, 2019		June 30, 2019		September 30, 2019		December 31, 2019
Loss from operations	$	(36,461)	$	(45,961)	$	(55,011)	$	(64,575)
Net loss	(31,586)		(41,243)		(50,735)		(61,030)
Net loss per share, basic and diluted	$	(0.32)	$	(0.41)	$	(0.50)	$	(0.58)

The following table provides the selected quarterly financial data for the year ended December 31, 2018 (in thousands, except per share amounts):


	Quarter Ended
	March 31, 2018		June 30, 2018		September 30, 2018		December 31, 2018
Loss from operations	$	2,597	$	135,012	$	22,187	$	33,046
Net loss	(2,597)		(134,902)		(43,497)		(30,509)
Net loss per share, basic and diluted	$	(0.10)	$	(43.82)	$	(10.71)	$	(0.37)

Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 0	$ 0	$ 0
State	0	1	2
Current federal, state and local, tax expense (benefit)	0	1	2
Deferred:
Federal	0	(251)	(89)
State	0	(81)	(30)
Deferred federal, state and local, tax expense (benefit)	0	(332)	(119)
Benefit for income taxes	$ 0	$ (331)	$ (117)

Stock-Based Compensation - Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions (Details) - Employee Stock Purchase Plan	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	60.40%	67.70%
Expected volatility, maximum	76.00%	81.80%
Expected risk-free interest rate, minimum	1.72%	2.37%
Expected risk-free interest rate, maximum	2.49%	2.83%
Expected dividend	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term in years	6 months	6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term in years	2 years	2 years

Investment (Tables)

12 Months Ended

Dec. 31, 2019

Investments, All Other Investments [Abstract]

Summary of Cash Equivalents and Available-for-Sale Securities

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2019 are presented in the following table:


	December 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
	(in thousands)
Money market funds	$	122,900	$	—	$	—	$	122,900
Corporate bonds	204,144		871		(4)		205,011
U.S. treasury securities	181,340		557		(3)		181,894
U.S. agency securities	25,658		167		(1)		25,824
Certificates of deposit	1,000		—		—		1,000
Total cash equivalents and investments	$	535,042	$	1,595	$	(8)	$	536,629

Classified as:
Cash equivalents							$	122,900
Short-term investments							355,407
Long-term investments							58,322
Total cash equivalents, and investments							$	536,629

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2018 are presented in the following table:


	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
	(in thousands)
Money market funds	$	61,023	$	—	$	—	$	61,023
Commercial Paper	4,917		—		—		4,917
Corporate bonds	244,136		220		(280)		244,076
U.S. treasury securities	341,696		342		(37)		342,001
U.S. agency securities	61,937		181		(3)		62,115
Total cash equivalents and investments	$	713,709	$	743	$	(320)	$	714,132

Classified as:
Cash equivalents							$	85,214
Short-term investments							366,952
Long-term investments							261,966
Total cash equivalents, and investments							$	714,132

Schedule of Available For Sale Debt Investments by Contractual Maturity

The fair values of available-for-sale debt investments by contractual maturity as of December 31, 2019 and 2018 were as follows:


	December 31,
	2019		2018
	(in thousands)
Due in 1 year or less	$	355,407	$	375,625
Due in 1 - 2 years	58,322		240,614
Due in 3 years	—		36,870
Instruments not due at a single maturity date	122,900		61,023
Total cash equivalents and investments	$	536,629	$	714,132

Balance Sheet Components

12 Months Ended

Dec. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Balance Sheet Components

Prepaid Expenses and Other Current Assets


	December 31,
	2019		2018
	(in thousands)
Prepaid research and development expenses	$	6,387	$	2,356
Prepaid insurance	2,625		2,376
Accrued interest on short-term marketable securities	2,403		3,108
Other prepaid and current assets	2,628		758
Total prepaid expenses and other current assets	$	14,043	$	8,598

Property and Equipment, Net


	December 31,
	2019		2018
	(in thousands)
Leasehold improvements	$	29,924	$	15
Laboratory equipment	13,117		5,534
Construction in progress	12,390		2,703
Computers equipment and purchased software	3,726		1,327
Furniture and fixtures	2,764		64
Total	61,921		9,643
Less: accumulated depreciation	(5,472)		(1,048)
Total property and equipment, net	$	56,449	$	8,595

Depreciation expense for the years ended December 31, 2019 and 2018 was $4.6 million and $1.0 million, respectively. Disposals of property and equipment were $0.2 million and zero for the years ended December 31, 2019 and 2018, respectively.

Intangible Assets, Net


	December 31, 2019
	Cost		Accumulated Amortization		Carrying Value
	(in thousands)
Assembled workforce	$	1,206	$	(1,055)	$	151


	December 31, 2018
	Cost		Accumulated Amortization		Carrying Value
	(in thousands)
Assembled workforce	$	1,206	$	(452)	$	754

As of December 31, 2019, the weighted-average remaining amortization period of the assembled workforce was 0.26 years. Amortization expense related to the assembled workforce intangible assets was $0.6 million and $0.5 million for the years ended December 31, 2019 and 2018, respectively.

Accrued Liabilities

Accrued liabilities consist of the following:


	December 31,
	2019		2018
	(in thousands)
Accrued compensation and related benefits	$	9,560	$	4,111
Accrued research and development expenses	4,833		7,808
Accrued property and equipment	3,575		—
Unvested shares liabilities	2,843		4,590
Other	3,018		612
Total accrued and other current liabilities	$	23,829	$	17,121

Investments - Schedule of Fair Values of Available For Sale Debt Investments by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Due in 1 year or less	$ 355,407	$ 375,625
Due in 1 - 2 years	58,322	240,614
Due in 3 years	0	36,870
Instruments not due at a single maturity date	122,900	61,023
Total cash equivalents and investments	$ 536,629	$ 714,132

Net Loss and Net Loss Per Share - Schedule of Anti-dilutive Shares (Details) - shares	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	27,948,931	32,087,884
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	9,190,522	7,235,545
Restricted stock units subject to vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	1,941,155	0
Expected shares purchased under Employee Stock Purchase Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	195,161	144,272
Founder shares subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	13,629,803	19,687,487
Early exercised stock options subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share (in shares)	0	2,992,290	5,020,580

Income Taxes - Summary of Operating Loss Carryforwards and Tax Credits (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Federal \| Indefinite
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	$ 193,991
Federal \| 2037
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	2
Federal \| 2038-2039
Operating Loss Carryforwards [Line Items]
Tax credits	4,037
State \| Indefinite
Operating Loss Carryforwards [Line Items]
Tax credits	3,832
State \| 2037-2039
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	$ 190,159