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Document And Entity Information - USD ($) $ in Thousands	12 Months Ended
Document And Entity Information - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 04, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Allogene Therapeutics, Inc.
Document Type	10-K
Trading Symbol	ALLO
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		121,482,671
Amendment Flag	false
Entity Central Index Key	0001737287
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Public Float			$ 0
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 92,432
Short-term investments	366,952
Prepaid expenses and other current assets	8,598
Total current assets	467,982
Long-term investments	261,966
Operating lease right-of-use asset	33,015
Property and equipment, net	8,595
Intangible assets, net	754
Restricted cash	1,299
Other long-term assets	244
Total assets	773,855
Current liabilities:
Accounts payable	12,338
Accrued and other current liabilities	17,121	$ 2
Total current liabilities	29,459	2
Lease liability, noncurrent	34,456
Other long-term liabilities	6,776
Total liabilities	70,691	2
Commitments and Contingencies (Notes 7, 8 and 9)
Convertible preferred stock, $0.001 par value; no shares and 1,000,000 shares authorized as of December 31, 2018 and December 31, 2017, respectively; no shares issued and outstanding as of December 31, 2018 and December 31, 2017
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value: 10,000,000 and no shares authorized as of December 31, 2018 and December 31, 2017, respectively; no shares were issued and outstanding as of December 31, 2018
Common stock, $0.001 par value: 200,000,000 and 47,250,000 shares authorized as of December 31, 2018 and December 31, 2017, respectively; 121,482,671 and 26,249,993 shares issued and outstanding as of December 31, 2018 and December 31, 2017, respectively	121	26
Notes receivable from common stockholders		(5)
Additional paid-in capital	914,265
Accumulated deficit	(211,528)	(23)
Accumulated other comprehensive income	306
Total stockholders’ equity (deficit)	703,164	$ (2)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 773,855

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	1,000,000
Convertible preferred stock, shares issued	0	0
Convertible preferred stock, shares outstanding	0	0
Preferred stock, par value per share	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0
preferred stock, shares outstanding	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	47,250,000
Common stock, shares issued	121,482,671	26,249,993
Common stock, shares outstanding	121,482,671	26,249,993

Condensed Statements of Operations and Comprehensive Loss - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Operating expenses:
Research and development		$ 151,860,000
General and administrative	$ 2,000	40,982,000
Total operating expenses	2,000	192,842,000
Loss from operations	(2,000)	(192,842,000)
Other income (expense), net:
Change in fair value of convertible note payable	0	(21,211,000)
Interest expense		(3,358,000)
Interest and other income, net		5,789,000
Total other income (expense), net		(18,780,000)
Loss before income taxes	(2,000)	(211,622,000)
Benefit from income taxes	0	117,000
Net loss	(2,000)	(211,505,000)
Other comprehensive loss:
Net unrealized gain on available-for-sale investments, net of tax		306,000
Net comprehensive loss	$ (2,000)	$ (211,199,000)
Net loss per share, basic and diluted	$ 0.00	$ (7.31)
Weighted-average number of shares used in computing net loss per share, basic and diluted	26,249,993	28,948,386

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, Shares at Nov. 29, 2017						0
Issuance of common stock upon initial public offering, net of issuance costs	5					$ 26					(21)
Issuance of common stock upon initial public offering, net of issuance costs, Shares						26,249,993
Notes receivable from common stockholders	(5)							(5)
Net loss and comprehensive loss	(2)										(2)
Balance at Dec. 31, 2017	$ (2)					$ 26		(5)			(23)
Balance, Shares at Dec. 31, 2017	26,249,993					26,249,993
Issuance of Series A convertible preferred shares at $35.06 pershare, net of issuance costsof $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, 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, 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 early exercise of stock options, Shares	5,020,580					5,020,580
Issuance of common stock upon initial public offering, net of issuance costs		$ 343,329					$ 21			$ 343,308
Issuance of common stock upon initial public offering, net of issuance costs, 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, 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, Shares		7,856,176				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					$ 121			$ 914,265		$ (211,528)	$ 306
Balance, Shares at Dec. 31, 2018	121,482,671					121,482,671

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

Statements of Cash Flows - USD ($)	1 Months Ended	12 Months Ended
Statements of Cash Flows - USD ($)	Dec. 31, 2017	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (2,000)	$ (211,505,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development	0	109,436,000
Stock-based compensation	0	18,566,000
Amortization of other intangible assets acquired	0	452,000
Depreciation and amortization	0	1,048,000
Net amortization/accretion on investment securities	0	(1,036,000)
Non-cash rent expense	0	1,832,000
Change in fair value of convertible notes payable	0	21,211,000
Debt issuance costs on convertible notes payable	0	3,358,000
Income tax benefit	0	(117,000)
Other	0	6,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	0	(8,598,000)
Other long-term assets	0	(244,000)
Accounts payable	0	8,800,000
Accrued and other current liabilities	2,000	12,138,000
Net cash used in operating activities	0	(44,653,000)
Cash flows from investing activities:
Purchases of property and equipment	0	(3,234,000)
Proceeds from sales of investments	0	2,606,000
Proceeds from maturities of investments	0	19,235,000
Purchase of investments	0	(649,307,000)
Cash paid for acquisition of assets	0	(2,098,000)
Net cash used in investing activities	0	(632,798,000)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	299,281,000
Proceeds from issuance of convertible notes, net of issuance costs	0	116,842,000
Proceeds from issuance of common stock and upon exercise of stock options	0	11,370,000
Proceeds from issuance of common stock, net of issuance costs	0	343,689,000
Net cash provided by financing activities	0	771,182,000
Net increase in cash, cash equivalents and restricted cash	0	93,731,000
Cash, cash equivalents and restricted cash — beginning of period	0	0
Cash, cash equivalents and restricted cash — end of period	0	93,731,000
Non-cash operating, investing and financing activities:
Common stock issued on conversion of convertible preferred stock	0	411,052,000
Common stock issued on conversion of convertible notes payable	0	141,410,000
Series A-1 convertible preferred stock issued in asset acquisition	0	111,770,000
PP&E and other assets acquired in asset acquisition	0	111,770,000
Right-of-use asset obtained in exchange for lease liability	0	33,015,000
Property and equipment purchases in accounts payable and accrued liabilities	0	3,182,000
Deferred offering costs included in accounts payable and accrued and other current liabilities	0	356,000
Supplemental disclosure:
Cash paid for amounts included in the measurement of lease liabilities	$ 0	$ (31,000)

Description of Business and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Disclosure Text Block [Abstract]

Description of Business and Summary of Significant Accounting Policies

Note 1. 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).

Initial Public Offering

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.

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. There were no deferred offering costs capitalized as of December 31, 2017.

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 $721.4 million as of December 31, 2018. Since inception through December 31, 2018, the Company has incurred cumulative net losses of $211.5 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 a period of at least one year from the date the financial statements are 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 accounting principles generally accepted in the United States of America (U.S. 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, 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, 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 industry 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. 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 loss. 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 operations.

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. 2016-02, 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.

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 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 year ended December 31, 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 the carrying amount of an asset group exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset group exceeds the fair value of the asset group. 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 has been no impairment of long-lived assets for any of the periods presented.

Research and Development Expenses

Research and development costs are expensed as incurred. Research and development expenses for the year ended December 31, 2018 primarily consist 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.

Research and development expenses also include costs incurred for internal and sponsored and collaborative research and development activities. Research and development costs 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. Costs associated with co-development activities performed under the various license and collaboration agreements are included in research and development expenses.

Recent Accounting Guidance

12 Months Ended

Dec. 31, 2018

Accounting Changes And Error Corrections [Abstract]

Recent Accounting Guidance

Note 2. 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 haven’t 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.

Recent Accounting Pronouncements Not Yet Adopted

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 does not expect the adoption of this standard to have a material effect on its financial statements.

In August 2018, the FASB issued Accounting Standards Update No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, which eliminates certain disclosure requirements for fair value measurements for all entities, requires public entities to disclose certain new information and modifies some disclosure requirements. This standard is effective for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of this standard to have a material effect on its 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 evaluating the impact of adopting this amendment to its financial statements.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

Note 3. Fair Value Measurements

The 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.

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

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

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 year ended December 31, 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

Investments

12 Months Ended

Dec. 31, 2018

Investments All Other Investments [Abstract]

Investments

Note 4. Investments

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, 2018 were as follows:

	Fair Value
	(in thousands)
Due in 1 year or less	$	375,625
Due in 1 - 2 years		240,614
Due in 3 years		36,870
Instruments not due at a single maturity date		61,023
Total cash equivalents and investments	$	714,132

As of December 31, 2018, the remaining contractual maturities of available-for-sale securities were less than three years. There have been no significant realized losses on available-for-sale securities for the year ended December 31, 2018. Based on our review of our available-for-sale securities, we believe we had no other-than-temporary impairments on these securities as of December 31, 2018, because we do not intend to sell these securities nor do we believe that we 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 year ended December 31, 2018.

Balance Sheet Components

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

Note 5. Balance Sheet Components

Prepaid Expenses and Other Current Assets

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

Property and Equipment, Net

	December 31,
	2018
	(in thousands)
Laboratory equipment	$	5,534
Leasehold improvements		15
Computers equipment and purchased software		1,327
Furniture and fixtures		64
Construction in progress		2,703
Total		9,643
Less: accumulated depreciation		(1,048	)
Total property and equipment, net	$	8,595

Depreciation and amortization expense for the year ended December 31, 2018 was $1.0 million.

Intangible Assets, Net

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

As of December 31, 2018, the weighted-average remaining amortization period of the assembled workforce was 1.26 years. Amortization expense related to the assembled workforce intangible assets was $0.5 million for the year ended December 31, 2018.

Accrued Liabilities

Accrued liabilities consist of the following:

	December 31, 2018		December 31, 2017
	(in thousands)
Accrued research and development expenses	$	7,808	$	—
Unvested shares liabilities		4,590	—
Accrued compensation and related benefits		4,111	—
Other		612		2
Total accrued and other current liabilities	$	17,121	$	2

Asset Acquisition

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Asset Acquisition

Note 6. 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 2017-01. 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 is 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 year ended December 31, 2018. These contingent payments are 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 is 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.

License and Collaboration Agreements

12 Months Ended

Dec. 31, 2018

Research And Development [Abstract]

License and Collaboration Agreements

Note 7. 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. No such payments were made in the year ended December 31, 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). Pursuant to the Original Cellectis Agreement, the Company has an exclusive, worldwide, royalty-bearing, sublicensable license, on a target-by-target basis, under certain of Cellectis’s intellectual property to make, use, sell, import, and otherwise commercialize products directed at certain targets for the treatment of cancer.

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. Pursuant to the terms of the Original Cellectis Agreement, the research collaboration ended in June 2018. Cellectis has 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 conduct research, and to make, use, sell, import and otherwise commercialize products directed at Cellectis-selected targets.

The Original Cellectis Agreement requires the Company to make payments of up to $185.0 million per product that is directed against a Company-selected target, with aggregate maximum potential pre-clinical, clinical and commercial milestone payments totaling up to $2.8 billion across all potential targets. 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, or are covered by, certain of Cellectis’s intellectual property at rates in the high single-digit percentages.

Unless earlier terminated in accordance with the agreement, the Original Cellectis Agreement will expire on a product-by-product and country-by-country basis, on 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 will the term extend, with respect to a particular licensed product, past the twentieth anniversary of the first commercial sale for such product.

All costs the Company incurred in connection with this agreement were recognized as research and development expenses. For the year ended December 31, 2018, $0.4 million 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 products, including other anti-CD19 product candidates.

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 generally not required to make any additional payments to Servier to exercise an option, except for products directed at a certain target, for which the Company is required to pay Servier an option fee in the low tens of millions of dollars range upon exercise. 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 and ALLO-501, 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 on a target-by-target basis. For products directed against CD19, including UCART19, 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. The total potential payments that the Company is obligated to make under the Servier Agreement upon successful completion of regulatory and sales milestones are $381.5 million, including the CD19-related milestone payments described above. 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 ($81.9 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 year ended December 31, 2018 the Company recorded development costs of $4.2 million incurred under the collaboration agreement with Servier. These costs were recognized as research and development expenses.

Leases

12 Months Ended

Dec. 31, 2018

Leases [Abstract]

Leases

Note 8. 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 begin 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 $24.6 million as of December 31, 2018 and an aggregate lease liability of $26.3 million in its balance sheet. The remaining lease term is 10 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 $6.2 million as of December 31, 2018 and an aggregate lease liability of $6.3 million in its balance sheet. The remaining lease term is 10 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 $2.0 million and $0.2 million as of December 31, 2018 and aggregate lease liabilities of $2.0 million and $0.2 million respectively for these leases. 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 two years which is not reasonably assured of exercise. There were not 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 6 years and 6 months and 2 years and 11 months at December 31, 2018 and the estimated incremental borrowing rates applied were 8% and 7%, respectively.

The undiscounted future non-cancellable lease payments under our operating leases as of December 31, 2018 is as follows:

Year ending December 31:	(in thousands)
2019	$	3,602
2020		5,653
2021		5,834
2022		5,942
2023 and thereafter		39,791
Total undiscounted lease payments		60,822
Less: Present value adjustment		(20,107	)
Less: Tenant improvement allowance		(5,942	)
Total	$	34,773

Rent expense for the Company’s operating leases was $1.9 million for the year ended December 31, 2018. Rent expense for short-term leases was $2.6 million for the year ended December 31, 2018. There was a total commitment of $1.6 million at December 31, 2018 related to short-term leases. Variable lease payments for operating expenses were immaterial for the year ended December 31, 2018.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

Note 9. 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.

Convertible Notes Payable (2018 Notes)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Convertible Notes Payable (2018 Notes)

Note 10. 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 year ended December 31, 2018, the Company recognized $21.2 million 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.

Convertible Preferred Stock and Stockholders' Deficit

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Convertible Preferred Stock and Stockholders’ Deficit

Note 11. Convertible Preferred Stock and Stockholders’ Equity (Deficit)

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, 2018.

Common Stock

Pursuant to the Amended and Restated Certificate of Incorporation filed on October 15, 2018, as amended, the Company was authorized to issue a total of 200,000,000 shares of common stock, of which 121,482,671 shares were issued and outstanding at December 31, 2018.

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, 2018, no dividends on common stock had been declared by the Company’s board of directors.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

Note 12. 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 31^st 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. 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, 2018, there were 8,176,125 shares reserved by the Company under the 2018 Plan for the future issuance of equity awards.

The following summarizes option activity under the 2018 Plan:

				Outstanding Options
	Shares available for Grant under 2018 Plan			Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
									(in years)		(in thousands)
Balance, December 31, 2017	—			—			$	—	—		$	—
Increase in shares reserved for issuance		20,432,250		—
Options granted		(12,336,975	)		12,336,975			5.47
Options exercised	—				(5,020,580	)		2.27				123,808
Options forfeited		80,850			(80,850	)		2.27
Balance, December 31, 2018		8,176,125			7,235,545		$	7.72		9.62	$	139,001
Exercisable, December 31, 2018					2,657,545		$	3.94		9.55	$	61,096
Vested and expected to vest, December 31, 2018					7,235,545		$	7.72		9.62	$	139,001

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 Capital Market on December 31, 2018. During the year ended December 31, 2018, the estimated weighted-average grant-date fair value of employee options granted was $3.75 per share. As of December 31, 2018, there was $42.8 million of unrecognized stock-based compensation related to unvested stock options, which is expected to be recognized over a weighted-average period of 3.5 years. No options had vested during the year ended December 31, 2018.

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, 2018

Fair value of common stock		$2.27 - $26.52
Expected term in years		5.99 to 6.25
Expected volatility		74.2% - 77.0%
Expected risk-free interest rate		2.74% - 2.99%
Expected dividend		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 year ended December 31, 2018, total stock-based compensation expense related to stock options was $3.3 million.

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. 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, 2018
Expected term (in years)	0.50 – 2.00
Volatility	67.7% - 81.8%
Risk-free interest rate	2.37% - 2.83%
Dividend yield		—

For the year ended December 31, 2018, total stock-based compensation expense related to ESPP was $0.4 million.

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 11, 24,230,750 shares of founders’ stock remained unvested at the modification date in April 2018. For the year ended December 31, 2018, $14.9 million of stock-based compensation expense was recognized related to the vesting of 6,562,506 shares of founder stock. At December 31, 2018 there was $44.6 million of unrecognized stock-based compensation expense related to 19,687,487 shares of unvested founders’ stock which is expected to be recognized over 3.2 years. 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, 2018
	(in thousands)
Research and development	$	1,657
General and administrative		16,909
Total stock-based compensation expense	$	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 year ended December 31, 2018 5,020,580 options were early exercised. As of December 31, 2018, there was $4.6 million recorded in accrued and other liabilities 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.

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

Note 13. Related Party Transactions

As of December 31, 2018, Pfizer held 21,976,484 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. For the year ended December 31, 2018 the costs incurred under the Pfizer TSA were $10.1 million, with $4.9 million recorded in general and administrative expense and $5.2 million recorded in research and development expense.

The Company also purchased certain lab supplies and services from Pfizer in connection with its research and development activities. For the year ended December 31, 2018 the total lab supplies and services purchased from Pfizer was $10.4 million, which were recorded as research and development expense.

As of December 31, 2018, the Company had amounts payable to Pfizer of $5.7 million, 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 was $0.6 million for the year ended December 31, 2018 and was included in general and administrative expenses.

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 was $0.3 million for the year ended December 31, 2018 and was included in general and administrative expenses.

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 pays Bellco $26,250 per month in arrears commencing June 2018 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 cost incurred for services provided, bonus and out-of-pocket expenses incurred under this consulting agreement were $0.5 million for the year ended December 31, 2018 and were included in general and administrative expenses.

Leases

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 $0.2 million at December 31, 2018.

401(k) Plan

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

401(k) Plan

Note 14. 401(k) Plan

In 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 $0.4 million related to matched contributions for the year ended December 31, 2018.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

Note 15. Income Taxes

For the period from November 30, 2017 (inception) to December 31, 2017, the Company recorded no income tax expense. For the year ended December 31, 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, (Inception) to December 31,
	2018			2017
Current:	(in thousands)
Federal	$	—		$	—
State		2		—
		2		—
Deferred:
Federal		(89	)	—
State		(30	)	—
		(119	)	—
Benefit for income taxes	$	(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, (Inception) to December 31,
	2018			2017
	(in thousands)
Tax benefit at federal statutory rate	$	(44,441	)	$	—
State taxes, net of federal benefit		(10,652	)	—
Stock-based compensation		3,629		—
Research tax credits		(708	)	—
Write-off of in-process R&D		5,247		—
Change in fair value of convertible notes		4,454		—
Change in valuation allowance		41,916		—
Other		438		—
Benefit for incomes taxes	$	(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, 2018			Period from November 30, (Inception) to December 31, 2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	16,437		$	—
Tax credit carryforwards		1,239		—
Intangibles		23,086		—
Accrued expenses		952		—
Lease liabilities		9,730		—
Stock based compensation		360		—
Other	—			—
Total deferred tax assets		51,804		—
Deferred tax liabilities:
Fixed assets		(531	)	—
Right of use leased assets		(9,239	)	—
Investments		(118	)	—
Other	—			—
Total deferred tax liabilities		(9,888	)	—
Net deferred tax assets		41,916		—
Valuation allowance		(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 $41.9 million during the year ended December 31, 2018.

The following table sets forth our federal and state NOL carryforwards and federal research and development tax credits as of December 31, 2018:


	Amount		Expiration
	(in thousands)
Net operating losses, federal	$	59,014	Indefinite
Net operating losses, federal	$	2	2037
Net operating losses, state	$	57,895	2037 - 2038
Tax credits, federal	$	1,180	2037 - 2038
Tax credits, state	$	1,120	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 year ended December 31, 2018, the Company recorded a tax benefit of $0.1 million 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, 2018		December 31, 2017
	(in thousands)
Balance at beginning of the year:
Additions based on tax positions related to current year	$	920	$	—
Additions to tax position of prior year	—		—
Balance at end of the year	$	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, 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 from inception.

Net Loss and Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss and Net Loss Per Share

Note 16. 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, (Inception) to December 31,
	2018			2017
Numerator:
Net loss	$	(211,505	)	$	(2	)
Denominator:
Weighted average common shares outstanding		28,948,386			26,249,993
Net loss per share, basic and diluted	$	(7.31	)	$	(0.00	)

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, (Inception) to December 31,
	2018		2017
Stock options to purchase common stock		7,235,545	—
Founder shares subject to future vesting		19,687,487	—
Early exercised stock options subject to future vesting		5,020,580	—
Total		31,943,612	—

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

Note 17. Subsequent Events

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. The lease has a term of fifteen years and eight months, and is expected to commence in March 2020. Upon certain conditions, the Company has two ten-year options to extend the lease. Subject to rent abatement for the second through nine months of the lease, the Company will be required to pay $159,150 per month for rent for the first twelve months of the lease term which will increase at a rate of 3% per year. The Company will be 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.

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, 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.

The material rights and obligations of the parties under the Cellectis Agreement are otherwise consistent with the material rights and obligations of the parties under the Original Cellectis Agreement.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Information [Abstract]

Selected Quarterly Financial Data (unaudited)

Note 18. Selected Quarterly Financial Data (unaudited)

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,			June 30,			September 30,			December 31,
	2018			2018			2018			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	)

Our loss from operations, net loss, and net loss per share, basic and diluted, for the period from November 30, 2017 (inception) to December 31, 2017, are contained in our accompanying Statements of Operations and Comprehensive Loss for the period from November 30, 2017 (inception) to December 31, 2017.

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

12 Months Ended

Dec. 31, 2018

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

Accrued Research and Development Costs

Income Taxes

Stock-Based Compensation

Net Loss Per Share

Comprehensive Loss

Definite-Lived Intangible Assets

Impairment of Long-Lived Assets

Research and Development Expenses

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

12 Months Ended

Dec. 31, 2018

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

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

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

	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

Investment (Tables)

12 Months Ended

Dec. 31, 2018

Investments All Other Investments [Abstract]

Summary of Cash Equivalents, Restricted Cash and Investments, Classified as 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, 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 Fair Values 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, 2018 were as follows:

	Fair Value
	(in thousands)
Due in 1 year or less	$	375,625
Due in 1 - 2 years		240,614
Due in 3 years		36,870
Instruments not due at a single maturity date		61,023
Total cash equivalents and investments	$	714,132

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Prepayments and Other Current Assets

Prepaid Expenses and Other Current Assets

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

Schedule of Property and Equipment, Net

Property and Equipment, Net

	December 31,
	2018
	(in thousands)
Laboratory equipment	$	5,534
Leasehold improvements		15
Computers equipment and purchased software		1,327
Furniture and fixtures		64
Construction in progress		2,703
Total		9,643
Less: accumulated depreciation		(1,048	)
Total property and equipment, net	$	8,595

Schedule of Intangible Assets, Net

Intangible Assets, Net

	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, 2018		December 31, 2017
	(in thousands)
Accrued research and development expenses	$	7,808	$	—
Unvested shares liabilities		4,590	—
Accrued compensation and related benefits		4,111	—
Other		612		2
Total accrued and other current liabilities	$	17,121	$	2

Asset Acquisition (Tables)

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Schedule of Fair Value of Assets Acquired Table Text Block

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

Leases (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018 is as follows:

Year ending December 31:	(in thousands)
2019	$	3,602
2020		5,653
2021		5,834
2022		5,942
2023 and thereafter		39,791
Total undiscounted lease payments		60,822
Less: Present value adjustment		(20,107	)
Less: Tenant improvement allowance		(5,942	)
Total	$	34,773

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity Under Plan

The following summarizes option activity under the 2018 Plan:

				Outstanding Options
	Shares available for Grant under 2018 Plan			Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
									(in years)		(in thousands)
Balance, December 31, 2017	—			—			$	—	—		$	—
Increase in shares reserved for issuance		20,432,250		—
Options granted		(12,336,975	)		12,336,975			5.47
Options exercised	—				(5,020,580	)		2.27				123,808
Options forfeited		80,850			(80,850	)		2.27
Balance, December 31, 2018		8,176,125			7,235,545		$	7.72		9.62	$	139,001
Exercisable, December 31, 2018					2,657,545		$	3.94		9.55	$	61,096
Vested and expected to vest, December 31, 2018					7,235,545		$	7.72		9.62	$	139,001

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, 2018

Fair value of common stock		$2.27 - $26.52
Expected term in years		5.99 to 6.25
Expected volatility		74.2% - 77.0%
Expected risk-free interest rate		2.74% - 2.99%
Expected dividend		0%

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, 2018
Expected term (in years)	0.50 – 2.00
Volatility	67.7% - 81.8%
Risk-free interest rate	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, 2018
	(in thousands)
Research and development	$	1,657
General and administrative		16,909
Total stock-based compensation expense	$	18,566

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Summary of Valuation Allowance

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, (Inception) to December 31,
	2018			2017
Current:	(in thousands)
Federal	$	—		$	—
State		2		—
		2		—
Deferred:
Federal		(89	)	—
State		(30	)	—
		(119	)	—
Benefit for income taxes	$	(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, (Inception) to December 31,
	2018			2017
	(in thousands)
Tax benefit at federal statutory rate	$	(44,441	)	$	—
State taxes, net of federal benefit		(10,652	)	—
Stock-based compensation		3,629		—
Research tax credits		(708	)	—
Write-off of in-process R&D		5,247		—
Change in fair value of convertible notes		4,454		—
Change in valuation allowance		41,916		—
Other		438		—
Benefit for incomes taxes	$	(117	)	$	—

Schedule of Deferred Tax Assets and Liabilities

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

	Year Ended December 31, 2018			Period from November 30, (Inception) to December 31, 2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	16,437		$	—
Tax credit carryforwards		1,239		—
Intangibles		23,086		—
Accrued expenses		952		—
Lease liabilities		9,730		—
Stock based compensation		360		—
Other	—			—
Total deferred tax assets		51,804		—
Deferred tax liabilities:
Fixed assets		(531	)	—
Right of use leased assets		(9,239	)	—
Investments		(118	)	—
Other	—			—
Total deferred tax liabilities		(9,888	)	—
Net deferred tax assets		41,916		—
Valuation allowance		(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, 2018:


	Amount		Expiration
	(in thousands)
Net operating losses, federal	$	59,014	Indefinite
Net operating losses, federal	$	2	2037
Net operating losses, state	$	57,895	2037 - 2038
Tax credits, federal	$	1,180	2037 - 2038
Tax credits, state	$	1,120	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, 2018		December 31, 2017
	(in thousands)
Balance at beginning of the year:
Additions based on tax positions related to current year	$	920	$	—
Additions to tax position of prior year	—		—
Balance at end of the year	$	920	$	—

Net Loss and Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

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, (Inception) to December 31,
	2018			2017
Numerator:
Net loss	$	(211,505	)	$	(2	)
Denominator:
Weighted average common shares outstanding		28,948,386			26,249,993
Net loss per share, basic and diluted	$	(7.31	)	$	(0.00	)

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, (Inception) to December 31,
	2018		2017
Stock options to purchase common stock		7,235,545	—
Founder shares subject to future vesting		19,687,487	—
Early exercised stock options subject to future vesting		5,020,580	—
Total		31,943,612	—

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018 (in thousands, except per share amounts):

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2018			2018			2018			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	)

Description of Business and Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended				12 Months Ended
	Oct. 10, 2018 USD ($) $ / shares shares	Oct. 01, 2018	Dec. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) shares	Oct. 31, 2018 USD ($)
Subsidiary Sale Of Stock [Line Items]
Date of incorporation								Nov. 30, 2017
Start up costs \| $			$ 2,000
Net proceeds \| $	$ 343,300
Convertible preferred stock, shares outstanding			0	0				0
Preferred stock outstanding				0				0
Deferred offering costs \| $			$ 0						$ 3,200
Common stock, stock split description		1-for-5.25
Common stock forward split ratio		0.1905
Cash and cash equivalents and marketable securities \| $				$ 721,400				$ 721,400
Net loss and comprehensive loss \| $			$ (2)	$ (30,509)	$ (43,497)	$ (134,902)	$ (2,597)	(211,505)
IPO
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares	20,700,000
Common stock price per share \| $ / shares	$ 18.00
Underwriting discounts and commissions \| $	$ 26,100
Offering costs \| $	$ 3,200							$ 29,272
Convertible preferred stock, shares outstanding	11,743,987
Number of 2018 Notes converted into common stock								7,856,176
Preferred stock outstanding	0
Over-Allotment Option
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares	2,700,000
Common Stock
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares			26,249,993
Number of 2018 Notes converted into common stock								7,856,176
Common Stock \| IPO
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares								20,700,000
Number of convertible preferred stock converted to shares of common stock	61,655,922
Number of 2018 Notes converted into common stock	7,856,176

Description of Business and Summary of Significant Accounting Policies - Schedule of Estimated Life of Assets (Details)	12 Months Ended
	Dec. 31, 2018
Laboratory Equipment
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	P5Y
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	P7Y
Leasehold Improvements
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	Shorter of lease term or useful life

Fair Value Measurements - Additional Information (Detail) - Fair Value, Measurements, Recurring	Dec. 31, 2018 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	$ 714,132,000
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Financial Liabilities	$ 0

Investments - Summary of Cash Equivalents, Restricted Cash and Investments, Classified as Available-for-Sale Securities (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 713,709
Unrealized Gains	743
Unrealized Losses	(320)
Fair Value	714,132
Cash equivalents	85,214
Short-term investments	366,952
Long-term investments	261,966
Total cash equivalents, and investments	714,132
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	61,023
Unrealized Gains	0
Unrealized Losses	0
Fair Value	61,023
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	4,917
Unrealized Gains	0
Unrealized Losses	0
Fair Value	4,917
Corporate Bond Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	244,136
Unrealized Gains	220
Unrealized Losses	(280)
Fair Value	244,076
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	341,696
Unrealized Gains	342
Unrealized Losses	(37)
Fair Value	342,001
U.S. Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	61,937
Unrealized Gains	181
Unrealized Losses	(3)
Fair Value	$ 62,115

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

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

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
Accrued interest on short-term marketable securities	$ 3,108
Prepaid insurance	2,376
Prepaid research and development expenses	2,356
Other prepaid and current assets	758
Total prepaid expenses and other current assets	$ 8,598

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	$ 9,643
Less: accumulated depreciation	(1,048)
Total property and equipment, net	8,595
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	5,534
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	15
Computer Equipment and Purchased Software
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	1,327
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	64
Construction In Progress
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	$ 2,703

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Balance Sheet Components [Line Items]
Depreciation and amortization expense		$ 1,000
Amortization expense	$ 0	$ 452
Assembled Workforce
Balance Sheet Components [Line Items]
Intangible assets, weighted-average remaining amortization period		1 year 3 months 3 days
Amortization expense		$ 500

Balance Sheet Components - Schedule of Intangible Assets (Details) - Assembled Workforce $ in Thousands	Dec. 31, 2018 USD ($)
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Intangible Assets, Cost	$ 1,206
Intangible Assets, Accumulated Amortization	(452)
Intangible Assets, Carrying Value	$ 754

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Accrued research and development expenses	$ 7,808
Unvested shares liabilities	4,590
Accrued compensation and related benefits	4,111
Other	612	$ 2
Total accrued and other current liabilities	$ 17,121	$ 2

Asset Acquisition - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Apr. 30, 2018	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		$ 0.0
Aggregate milestone payments	$ 840.0
Pfizer \| Asset Contribution Agreement \| UNITED STATES
Asset Acquisition [Line Items]
Milestone payments, per target	30.0
Pfizer \| Asset Contribution Agreement \| European Union
Asset Acquisition [Line Items]
Milestone payments, per target	$ 60.0
Series A1 Convertible Preferred Shares
Asset Acquisition [Line Items]
Share issued in connection with asset acquisition	3,187,772
Share issued in connection with asset acquisition, fair value	$ 111.8
Convertible preferred stock, price per share	$ 35.06
Costs from issuance of preferred shares	$ 2.1
Total asset acquisition	$ 113.9

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

License and Collaboration Agreements - Additional Information (Details) - 12 months ended Dec. 31, 2018 € in Millions	USD ($)	EUR (€)
License Agreements [Line Items]
Research and development	$ 151,860,000
License and Collaboration Agreement
License Agreements [Line Items]
Percentage of development cost responsible for	60.00%
Pfizer \| Asset Contribution Agreement \| UNITED STATES
License Agreements [Line Items]
Milestone payments, per target	$ 30,000,000
Pfizer \| Asset Contribution Agreement \| European Union
License Agreements [Line Items]
Milestone payments, per target	60,000,000
Pfizer \| Asset Contribution Agreement \| North America, Europe, Asia, Australia and Oceania
License Agreements [Line Items]
Aggregate potential milestone payments	325,000,000
Pfizer \| Asset Contribution Agreement \| Maximum \| European Union
License Agreements [Line Items]
Aggregate potential regulatory and development milestones	840,000,000
Cellectis \| Research Collaboration and License Agreement
License Agreements [Line Items]
Maximum payments required per product against selected target	185,000,000
Research and development	400,000
Cellectis \| Maximum \| Research Collaboration and License Agreement
License Agreements [Line Items]
Aggregate potential milestone payments	2,800,000
Servier \| License and Collaboration Agreement
License Agreements [Line Items]
Research and development	$ 4,200,000
Percentage of development cost responsible for	40.00%
Servier \| Regulatory Milestone \| License and Collaboration Agreement
License Agreements [Line Items]
Aggregate potential milestone receivable	$ 42,000,000
Servier \| Regulatory Milestone \| Maximum \| License and Collaboration Agreement
License Agreements [Line Items]
Aggregate potential milestone payments	137,500,000
Servier \| Sales Milestone \| License and Collaboration Agreement
License Agreements [Line Items]
Aggregate potential milestone receivable	81,900,000	€ 70.5
Servier \| Sales Milestone \| Maximum \| License and Collaboration Agreement
License Agreements [Line Items]
Aggregate potential milestone payments	78,000,000
Servier \| Regulatory and Sales Milestone \| License and Collaboration Agreement
License Agreements [Line Items]
Payments to milestone	$ 381,500,000

Leases - Additional Information (Details)	1 Months Ended		12 Months Ended
Leases - Additional Information (Details)	Oct. 31, 2018 USD ($) Sq.Ft	Aug. 31, 2018 USD ($) Sq.Ft	Dec. 31, 2018 USD ($) Sq.Ft OperatingLease
Operating Leased Assets [Line Items]
Area of office and laboratory \| Sq.Ft	14,943	68,000
Operating lease term	124 months	127 months
Operating lease expiration period	Feb. 28, 2029	Feb. 28, 2029
Operating lease, option to extend term	7 years	7 years
Allowance for tenant improvements	$ 800,000	$ 5,100,000	$ 5,942,000
Letter of credit	$ 200,000	$ 1,000,000
Operating lease right-of-use asset			33,015,000
Aggregate lease liability, noncurrent			$ 34,456,000
Number of operating Leases for office space \| OperatingLease			2
Rent expense			$ 1,900,000
Rent expense for short-term leases			2,600,000
Short tem lease total commitment			$ 1,600,000
New York
Operating Leased Assets [Line Items]
Area of office and laboratory \| Sq.Ft			4,358
Operating lease term			6 years 7 months
Allowance for tenant improvements			$ 0
Letter of credit			100,000
Operating lease right-of-use asset			2,000,000
Aggregate lease liability, noncurrent			$ 2,000,000
Estimated incremental borrowing rate of lease			8.00%
Remaining Lease term			6 years 6 months
Los Angeles
Operating Leased Assets [Line Items]
Area of office and laboratory \| Sq.Ft			1,293
Operating lease term			3 years
Operating lease, option to extend term			2 years
Allowance for tenant improvements			$ 0
Operating lease right-of-use asset			200,000
Aggregate lease liability, noncurrent			$ 200,000
Estimated incremental borrowing rate of lease			7.00%
Remaining Lease term			2 years 11 months
127 Months Lease Term
Operating Leased Assets [Line Items]
Operating lease right-of-use asset			$ 24,600,000
Aggregate lease liability, noncurrent			$ 26,300,000
Remaining Lease term			10 years 2 months
Estimated incremental borrowing rate of lease			8.00%
124 Months Lease Term
Operating Leased Assets [Line Items]
Operating lease right-of-use asset			$ 6,200,000
Aggregate lease liability, noncurrent			$ 6,300,000
Remaining Lease term			10 years 2 months
Estimated incremental borrowing rate of lease			8.00%

Leases - Summary of Undiscounted Future Non-Cancellable Lease Payments Under Operating Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Oct. 31, 2018	Aug. 31, 2018
Operating Leases Future Minimum Payments Due [Abstract]
2019	$ 3,602
2020	5,653
2021	5,834
2022	5,942
2023 and thereafter	39,791
Total undiscounted lease payments	60,822
Less: Present value adjustment	(20,107)
Less: Tenant improvement allowance	(5,942)	$ (800)	$ (5,100)
Total	$ 34,773

Convertible Notes Payable (2018 Notes) - Additional information (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2018
Short Term Debt [Line Items]
Aggregate principal amount		$ 120,200
Proceeds from issuance of convertible notes, net of issuance costs	$ 0	116,800	$ 116,842
Fair value of notes		$ 120,200	141,400
Change in fair value of convertible notes payable			21,200
Debt issuance of cost			$ 3,400
IPO
Short Term Debt [Line Items]
Number of 2018 Notes converted into common stock			7,856,176

Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Apr. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Oct. 10, 2018	Aug. 31, 2018	Nov. 29, 2017
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		0	0
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 0	$ 299,281
Aggregate purchase price, shares value	$ 300,000
Subscriptions receivable					$ 150,000
Preferred stock, shares authorized		0	10,000,000
Preferred stock, shares issued			0
preferred stock, shares outstanding			0
Common stock, shares authorized		47,250,000	200,000,000
Common stock, shares issued		26,249,993	121,482,671
Common stock, shares outstanding		26,249,993	121,482,671
Modified shares of common stock			26,249,993
Compensation cost resulting from the modification			$ 59,500
Vesting term			4 years
Dividends declared on common stock			$ 0
IPO
Class Of Stock [Line Items]
Common stock price per share				$ 18.00
preferred stock, shares outstanding				0
Common Stock
Class Of Stock [Line Items]
Common stock, shares outstanding		26,249,993	121,482,671			0
Common Stock \| IPO
Class Of Stock [Line Items]
Number of convertible preferred stock converted to shares of common stock				61,655,922
Series A1 Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible preferred stock, shares issued	3,187,772
Convertible preferred stock, shares issued	998,225
Common stock price 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	7,557,990
Common stock price per share	$ 35.06
Proceeds from issuance of convertible preferred stock, net of issuance costs	$ 264,400

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2018	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		$ 18,566
Issuance of common stock for early exercise of stock options, Shares		5,020,580
Accrued and other liabilities, related to shares held by employees and directors that were subject to repurchase		$ 4,600
Other long term liabilities, related to shares held by employees and directors that were subject to repurchase		6,800
Founders
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock based compensation		$ 44,600
Weighted-average period recognized		3 years 2 months 12 days
Number of vested shares		6,562,506
Share-based compensation expense		$ 14,900
Number of shares unvested		19,687,487	24,230,750
Period of recognized compensation cost		3 years 2 months 12 days
Weighted average fair value at grant date		$ 2.27
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated weighted average grant date fair value of employee options granted		$ 3.75
Unrecognized stock based compensation		$ 42,800
Weighted-average period recognized		3 years 6 months
Number of vested shares		0
Period of recognized compensation cost		3 years 6 months
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares as percentage of common shares outstanding		5.00%
Share-based compensation arrangement by share-based payment award, Description		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. 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
Vesting term		4 years
Number of shares reserved for future issuance		8,176,125
Share-based compensation expense		$ 3,300
Issuance of common stock for early exercise of stock options, Shares		5,020,580
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%
2018 Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Combined voting power by individual		10.00%
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense		$ 400
Initially reserved common stock for employee purchase	1,160,000
Current offering period	24 months
Current offering description	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
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%

Stock-Based Compensation - Summary of Stock Option Activity Under Plan (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Options exercised	(5,020,580)
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for Grant, Balance	0
Shares available for Grant, Increase in shares reserved for issuance	20,432,250
Shares available for Grant, Options granted	(12,336,975)
Shares available for Grant, Options forfeited	80,850
Shares available for Grant, Balance	8,176,125
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Balance	0
Number of Options, Options granted	12,336,975
Number of Options, Options exercised	(5,020,580)
Number of Options, Options forfeited	(80,850)
Number of Options, Balance	7,235,545
Number of Options, Exercisable	2,657,545
Number of Options, Vested and expected to vest	7,235,545
Weighted-Average Exercise Price, Balance \| $ / shares	$ 0
Weighted-Average Exercise Price, Options granted \| $ / shares	5.47
Weighted-Average Exercise Price, Options exercised \| $ / shares	2.27
Weighted-Average Exercise Price, Options forfeited \| $ / shares	2.27
Weighted-Average Exercise Price, Balance \| $ / shares	7.72
Weighted-Average Exercise Price, Exercisable \| $ / shares	3.94
Weighted-Average Exercise Price, Vested and expected to vest \| $ / shares	$ 7.72
Weighted-Average Remaining Contract Term, Balance	9 years 7 months 13 days
Weighted-Average Remaining Contract Term, Exercisable	9 years 6 months 18 days
Weighted-Average Remaining Contract Term, Vested and expected to vest	9 years 7 months 13 days
Aggregate Intrinsic Value, Exercised \| $	$ 123,808
Aggregate Intrinsic Value, Balance \| $	139,001
Aggregate Intrinsic Value, Exercisable \| $	61,096
Aggregate Intrinsic Value, Vested and expected to vest \| $	$ 139,001

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

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, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	67.70%
Expected volatility, maximum	81.80%
Expected risk-free interest rate, minimum	2.37%
Expected risk-free interest rate, maximum	2.83%
Expected dividend	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term in years	6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term in years	2 years

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 18,566
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	1,657
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 16,909

Related Party Transactions - Additional Information (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2018 USD ($) Sq.Ft	Dec. 31, 2018 USD ($) Sq.Ft Director shares	Oct. 31, 2018 Sq.Ft	Jun. 30, 2018	Dec. 31, 2017 shares
Related Party Transaction [Line Items]
Shares held \| shares		121,482,671			26,249,993
Area of office and laboratory \| Sq.Ft	68,000		14,943
Operating lease term	127 months		124 months
Operating lease right-of-use asset		$ 33,015
Pfizer
Related Party Transaction [Line Items]
Shares held \| shares		21,976,484
Number of members appointed to board of directors \| Director		1
Payable to related party, current		$ 5,700
Pfizer \| Transition Services Agreement
Related Party Transaction [Line Items]
Related party costs		10,100
Related party general and administrative expense		4,900
Pfizer \| Research and Development Expense \| Transition Services Agreement
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		5,200
Pfizer \| Research and Development Expense \| Transition Services Agreement \| Lab Supplies
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		10,400
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		300
Ownership percentage				5.00%
Two River Consulting LLC \| General and administrative \| Affiliated with President and Chief Executive Officer
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		$ 600
Bellco \| Los Angeles California
Related Party Transaction [Line Items]
Area of office and laboratory \| Sq.Ft		1,293
Operating lease term		3 years
Operating lease right-of-use asset		$ 200
Bellco \| Consulting Agreements
Related Party Transaction [Line Items]
Related party transaction monthly payment in arrears	$ 26,250
Related party transaction agreement commencement date	Jun. 30, 2018
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		$ 500

401(k) Plan - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
K Plan [Abstract]
Contribution Expenses	$ 0.4

Income Taxes - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax benefit	$ 0	$ (117,000)
Increase in valuation allowance		41,900,000
Tax benefit		100,000
Accrued interest and penalties		$ 0

Income Taxes - Summary of Valuation Allowance (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Current:
State		$ 2,000
Current federal, state and local, tax expense (benefit)		2,000
Deferred:
Federal		(89,000)
State		(30,000)
Deferred federal, state and local, tax expense (benefit)		(119,000)
Benefit for income taxes	$ 0	$ (117,000)

Income Taxes - Schedule of Income Tax Renconciliation (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate		$ (44,441,000)
State taxes, net of federal benefit		(10,652,000)
Stock-based compensation		3,629,000
Research tax credits		(708,000)
Write-off of in-process R&D		5,247,000
Change in fair value of convertible notes		4,454,000
Change in valuation allowance		41,916,000
Other		438,000
Benefit for income taxes	$ 0	$ (117,000)

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Deferred tax assets:
Net operating loss carryforwards	$ 16,437
Tax credit carryforwards	1,239
Intangibles	23,086
Accrued expenses	952
Lease liabilities	9,730
Stock based compensation	360
Total deferred tax assets	51,804
Deferred tax liabilities:
Fixed assets	(531)
Right of use leased assets	(9,239)
Investments	(118)
Total deferred tax liabilities	(9,888)
Net deferred tax assets	41,916
Valuation allowance	$ (41,916)

Income Taxes - Summary of Operating Loss Carryforwards and Tax Credits (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Federal \| Indefinite
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	$ 59,014
Federal \| 2037
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	2
Federal \| 2037-2038
Operating Loss Carryforwards [Line Items]
Tax credits	1,180
State \| Indefinite
Operating Loss Carryforwards [Line Items]
Tax credits	1,120
State \| 2037-2038
Operating Loss Carryforwards [Line Items]
Net operating losses carryforwards	$ 57,895

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Income Tax Disclosure [Abstract]
Balance at beginning of the year:	$ 0
Additions based on tax positions related to current year	920
Additions to tax position of prior year	0
Balance at end of the year	$ 920

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
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Numerator:
Net loss	$ (2)	$ (30,509)	$ (43,497)	$ (134,902)	$ (2,597)	$ (211,505)
Denominator:
Weighted average common shares outstanding	26,249,993					28,948,386
Net loss per share, basic and diluted	$ 0.00	$ (0.37)	$ (10.71)	$ (43.82)	$ (0.10)	$ (7.31)

- Definition

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

+ References + Details

Net Loss and Net Loss Per Share - Schedule of Anti-dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details)	12 Months Ended
	Dec. 31, 2018 shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	31,943,612
Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	7,235,545
Founder Shares of Common Stock 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	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	5,020,580

Subsequent Events - Additional Information (Details)	1 Months Ended
Subsequent Events - Additional Information (Details)	Feb. 28, 2019 USD ($) ft² Renewal	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($)	Aug. 31, 2018 USD ($)
Subsequent Event [Line Items]
Operating lease term			124 months	127 months
Allowance for tenant improvements		$ 5,942,000	$ 800,000	$ 5,100,000
Letter of credit			$ 200,000	$ 1,000,000
Operating lease, option to extend term			7 years	7 years
Subsequent Event
Subsequent Event [Line Items]
Area of operating lease \| ft²	118,000
Operating lease term	15 years 8 months
Allowance for tenant improvements	$ 2,900,000
Letter of credit	$ 3,000,000
Operating lease commencement date	Mar. 31, 2020
Lease, Option to extend	Upon certain conditions, the Company has two ten-year options to extend the lease
Number of options to extend lease \| Renewal	2
Operating lease, option to extend term	10 years
Payment of lease rental per month	$ 159,150
Lessee, Operating Lease, Variable Lease Payment, Terms and Conditions	Subject to rent abatement for the second through nine months of the lease, the Company will be required to pay $159,150 per month for rent for the first twelve months of the lease term which will increase at a rate of 3% per year.
Increase in rate for rent per year for lease term	3.00%

Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Loss from operations	$ (2)	$ 33,046	$ 22,187	$ 135,012	$ 2,597	$ (192,842)
Net loss	$ (2)	$ (30,509)	$ (43,497)	$ (134,902)	$ (2,597)	$ (211,505)
Net loss per share, basic and diluted	$ 0.00	$ (0.37)	$ (10.71)	$ (43.82)	$ (0.10)	$ (7.31)

- Definition

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

+ References + Details