Allogene Therapeutics (ALLO) 10-Q 21 Nov 18 2018 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 15, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Allogene Therapeutics, Inc.
Document Type	10-Q
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	1,737,287
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Ex Transition Period	false

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 83,141
Short-term investments		161,080
Prepaid expenses and other current assets		3,132
Total current assets		247,353
Long-term investments		154,045
Operating lease right-of-use asset		24,996
Property and equipment, net		4,693
Intangible assets, net		904
Restricted cash		937
Other long-term assets		2,307
Total assets		435,235
Current liabilities:
Accounts payable		3,083
Accrued and other current liabilities		16,779	$ 2	[1]
Convertible notes payable		139,615
Total current liabilities		159,477	2	[1]
Lease liability, noncurrent		25,661
Other long-term liabilities		7,488
Total liabilities		192,626	2	[1]
Commitments and Contingencies (Note 7 and 8)				[1]
Convertible preferred stock, $0.001 par value; 11,743,987 and 1,000,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 11,743,987 shares and no shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively; aggregate liquidation preference of $411.8 million as of September 30, 2018		411,052
Stockholders’ deficit:
Common stock, $0.001 par value: 101,000,000 and 47,250,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 31,270,573 and 26,249,993 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively		31	26	[1]
Notes receivable from common stockholders	[1]		(5)
Additional paid-in capital		12,693
Accumulated deficit		(181,019)	(23)	[1]
Accumulated other comprehensive loss		(148)
Total stockholders’ deficit		(168,443)	$ (2)	[1]
Total liabilities, convertible preferred stock and stockholders’ deficit		$ 435,235
[1]	The balance sheet as of December 31, 2017 is derived from the audited financial statements as of that date.

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	11,743,987	1,000,000
Convertible preferred stock, shares issued	11,743,987	0
Convertible preferred stock, shares outstanding	11,743,987	0
Convertible preferred stock, aggregate liquidation Preference	$ 411,770
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	101,000,000	47,250,000
Common stock, shares issued	31,270,573	26,249,993
Common stock, shares outstanding	31,270,573	26,249,993

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Operating expenses:
Research and development	$ 10,870	$ 133,356
General and administrative	11,317	26,440
Total operating expenses	22,187	159,796
Loss from operations	(22,187)	(159,796)
Other income (expense), net:
Change in fair value of convertible note payable	(19,415)	(19,415)
Interest expense	(3,358)	(3,358)
Interest and other income, net	1,463	1,573
Total other income (expense), net	(21,310)	(21,200)
Net loss	(43,497)	(180,996)
Other comprehensive loss:
Net change in available-for-sale investments	(148)	(148)
Net comprehensive loss	$ (43,645)	$ (181,144)
Net loss per share, basic and diluted	$ (10.71)	$ (16.38)
Weighted-average number of shares used in computing net loss per share, basic and diluted	4,060,419	11,048,451

Condensed Statement of Cash Flo

Condensed Statement of Cash Flows $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Cash flows from operating activities:
Net loss	$ (180,996)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development	109,436
Stock-based compensation	12,695
Amortization of other intangible assets acquired	302
Depreciation and amortization	651
Net amortization/accretion on investment securities	125
Non-cash rent expense	664
Change in fair value of convertible notes payable	19,415
Debt issuance costs on convertible notes payable	3,358
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,132)
Other long-term assets	(1,468)
Accounts payable	3,083
Accrued and other current liabilities	12,695
Net cash used in operating activities	(23,172)
Cash flows from investing activities:
Purchases of property and equipment	(1,913)
Purchase of investments	(315,399)
Cash paid for acquisition of assets	(2,098)
Net cash used in investing activities	(319,410)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	299,282
Proceeds from issuance of convertible notes, net of issuance costs	116,842
Proceeds from issuance of common stock and upon exercise of stock options	11,375
Payments of deferred offering costs	(839)
Net cash provided by financing activities	426,660
Net increase in cash, cash equivalents and restricted cash	84,078
Cash, cash equivalents and restricted cash — end of period	84,078
Non-cash investing and financing activities:
Series A-1 convertible preferred stock issued in asset acquisition	111,770
Right-of-use asset obtained in exchange for lease liability	25,322
Deferred offering costs included in accounts payable and accrued and other current liabilities	1,388
Property and equipment purchase in accounts payable and accrued liabilities	$ 204

Description of Business

Description of Business	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business 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 10). 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 approximately $343.0 million in net proceeds, after deducting underwriting discounts and commissions of $26.1 million and offering expenses of approximately $3.5 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 9) were automatically converted into 7,856,176 shares of common stock. Following the IPO, there were no shares of preferred stock outstanding. As of September 30, 2018, the Company had incurred $2.2 million of deferred offering costs, which will be offset against the net proceeds received from the sale of common stock. 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. The Company had cash and cash equivalents and investments of $398.3 million as of September 30, 2018. Since inception through September 30, 2018, the Company has incurred cumulative net losses of $181.0 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 unaudited condensed financial statements are filed with the Securities and Exchange Commission (SEC). 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 condensed financial statements have been retrospectively adjusted to reflect the effect of the Forward Stock Split for all periods presented.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) for interim financial information and pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the Company’s opinion, all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation of the results of operations and cash flows for the periods presented have been included. The condensed balance sheet as of September 30, 2018, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2018, the condensed statement of cash flows for the nine months ended September 30, 2018, and the financial data and other financial information disclosed in the notes to the condensed financial statements are unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. These condensed financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated October 10, 2018 that forms a part of the Company's Registration Statements on Form S-1 (File Nos. 333-227333 and 333-227774), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended, on October 11, 2018. 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 convertible notes payable, 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 dictate. Actual results could differ from those estimates. 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 a letter of credit under a lease agreement which has been collateralized by restricted cash. This cash is classified as long-term restricted cash on the accompanying condensed balance sheet based on the term of the underlying lease. Investments Investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. 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. 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. As of September 30, 2018, $2.2 million of deferred offering costs have been capitalized, which is included in other long-term assets in the accompanying condensed balance sheet. There were no deferred offering costs recorded as of December 31, 2017. Leases The Company early adopted Accounting Standards Update (ASU) No. 2016-02, Leases as of January 1, 2018 in accordance with ASC 250, Accounting Changes and Error Corrections For its long-term operating 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 nonlease 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 its condensed balance sheets and within research and development expenses on the statements of operations. 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 in 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. Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses for the nine months ended September 30, 2018 primarily consist of acquired intangible assets as research and development costs pursuant to the Asset Contribution Agreement with Pfizer (see Note 7) 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 nine months ended September 30, 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. Recently Adopted Accounting Pronouncements In March 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2016-09, Stock Compensation—Improvements to Employee Share-Based Payment Accounting In January 2017, the FASB issued Accounting Standards Update, Business Combinations (Topic 805): Clarifying the Definition of a Business In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In November 2016, the FASB issued Accounting Standards Update No. Statement of Cash Flows: Restricted Cash 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. . The amendments in this ASU should be applied retrospectively to all periods presented. The Company The adoption of this ASU Leases The Company early adopted Accounting Standards Update (ASU) No. 2016-02, Leases as of January 1, 2018 in accordance with ASC 250, Accounting Changes and Error Corrections commenced on August 1, 2018. Recent Accounting Pronouncements Not Yet Adopted In February 2018, the FASB issued Accounting Standards Update No. Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In August 2018, the FASB issued Accounting Standards Update No. Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Disclosures	3. Fair Value Measurements 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. I nvestments 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. Level 3 liabilities consist of the convertible note payable. The determination of the fair value of the convertible note payable and the change in the fair value of the convertible notes payable is discussed in Note 9. Financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type as of September 30, 2018 are presented in the following tables: September 30, 2018 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 6,524 $ — $ — $ 6,524 Corporate bonds — 163,319 — 163,319 U.S. treasury securities 224,947 — — 224,947 Total financial assets $ 231,471 $ 163,319 $ — $ 394,790 Financial Liabilities: Convertible notes payable $ — $ — $ 139,615 $ 139,615 Total financial liabilities $ — $ — $ 139,615 $ 139,615 There were no transfers between Levels 1, 2 or 3 for the period presented.

Financial Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2018
Investments All Other Investments [Abstract]
Financial Instruments	4. Financial Instruments The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of September 30, 2018 are presented in the following table: September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 6,524 $ — $ — $ 6,524 Corporate bonds 163,413 18 (112 ) 163,319 U.S. treasury securities 225,001 — (54 ) 224,947 Total cash equivalents, restricted cash and investments $ 394,938 $ 18 $ (166 ) $ 394,790 Classified as: Cash equivalents $ 78,728 Restricted cash 937 Short-term investments 161,080 Long-term investments 154,045 Total cash equivalents, restricted cash and investments $ 394,790 As of September 30, 2018, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized losses on available-for-sale securities for the period presented. Available-for-sale debt securities that were in a continuous loss position but were not deemed to be other than temporarily impaired were immaterial as of September 30, 2018.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	5 . Balance Sheet Components Intangible Assets, Net September 30, 2018 Cost Accumulated Amortization Carrying Value (in thousands) Assembled workforce $ 1,206 $ (302 ) $ 904 As of September 30, 2018, the weighted-average remaining amortization period of the assembled workforce was 1.51 years. Amortization expense related to the assembled workforce was $0.2 million and $0.3 million for the three and nine months ended September 30, 2018, respectively. Accrued Liabilities Accrued liabilities consist of the following: September 30, 2018 December 31, 2017 (In thousands) Accrued research and development expenses $ 6,584 $ — Unvested shares liabilities 3,883 — Accrued compensation and related benefits 1,728 — Accrued offering costs 579 — Accrued professional and consulting services 2,066 — Other 1,939 2 Total accrued and other current liabilities $ 16,779 $ 2

Asset Acquisition

Asset Acquisition	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Asset Acquisition	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 of $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 10 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 off 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. 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 Agreements

License Agreements	9 Months Ended
	Sep. 30, 2018
Research And Development [Abstract]
License Agreements	7 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. 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 Cellectis Agreement) with Cellectis S.A. (Cellectis). Pursuant to the 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 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 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 Cellectis Agreement requires Allogene 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 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 nine months ended September 30, 2018, $0.4 million of costs have been incurred associated with research services performed by Cellectis. For the three months ended September 30, 2018, no cost has been incurred associated with research services performed by Cellectis. As of September 30, 2018, no amount was recorded in the accrued and other current liabilities in the accompanying condensed balance sheet. 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, the Company is responsible for 60% of the development costs and Servier is responsible for the remaining 40% of the development costs under the 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 three and nine months ended September 30, 2018, the Company recorded $4.8 million and $7.5 million of the costs incurred under the cost-sharing terms of the Servier Agreement as research and development expenses. As of September 30, 2018, $4.6 million was recorded in the accrued and other current liabilities in the accompanying condensed balance sheet

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	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. In connection with the lease, the Company has maintained a letter of credit for the benefit of the landlord in the amount of $0.9 million In connection with the lease, the Company recognized an operating lease right-of-use asset of $25.0 million as of September 30, 2018 and an aggregate lease liability of $25.7 million in its condensed balance sheet. The undiscounted future lease payments under the lease liability as of September 30, 2018 were as follows: Year ending December 31: (in thousands) 2018 (remaining three months) $ — 2019 2,406 2020 4,245 2021 4,394 2022 4,548 2023 and thereafter 31,769 Total undiscounted lease payments 47,362 Less: Present value adjustment (16,596 ) Less: Tenant improvement allowance (5,105 ) Total $ 25,661 Rent expense for the operating lease was $0.7 million for both the three and nine months ended September 30, 2018, respectively. Short-term lease expense was $1.0 million and $1.7 million for the three and nine months ended September 30, 2018, respectively. Variable lease payments for operating expenses were immaterial for the three and nine months ended September 30, 2018.

Convertible Notes Payable (2018

Convertible Notes Payable (2018 Notes)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Convertible Notes Payable (2018 Notes)	9 . 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. 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. The 2018 Notes also contained other settlement provisions if an IPO did not occur within one year of the 2018 Notes issuance date. If the Company was acquired, completed a business combination resulting in a change of control or sold all or substantially all of its assets (each, a “liquidation transaction”) prior to the one-year anniversary of the issuance date of the 2018 Notes, the 2018 Notes would settle into shares of common stock at a price per share equal to 85% of the estimated fair value of the consideration per share payable to the holders of common stock in connection with such liquidation transaction. If neither the IPO nor a liquidation transaction occurred prior to the one-year anniversary of the issuance date of the 2018 Notes, the 2018 Notes would convert into shares of the Company’s newly designated Series B convertible preferred stock at a settlement price per share that would be determined based on a stipulated $900.0 million valuation of the Company and its fully diluted capitalization as of immediately prior to the conversion of the 2018 Notes. The 2018 Notes also contained additional redemption features contingent upon the occurrence of certain future events. On issuance, the Company elected to account for the 2018 Notes at fair value with any changes in fair value being recognized through the statements of operations until the 2018 Notes settled. The fair value of the 2018 Notes was determined to be $120.2 million on issuance and $139.6 million as of September 30, 2018. For the three-months and nine-months ended September 30, 2018, the Company recognized $19.4 million in the accompanying condensed statements of operations as 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 condensed statements of operations. Fair Value 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 Company determined the fair value of the 2018 Notes as of September 30, 2018 by determining the present value of the 2018 Notes if they were to settle either upon the closing of an IPO or in shares of Series B convertible preferred stock one year after issuance (September 2019). Key valuation assumptions are as follows: As of September 30, 2018 IPO Scenario Series B Scenario Expected term (years) 0.03 – 0.10 1 year Discount rate 20% — Weighting 95% 5%

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Deficit	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Convertible Preferred Stock and Stockholders’ Deficit	10 . Convertible Preferred Stock and Stockholders’ 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. Convertible preferred stock consisted of the following: September 30, 2018 Shares Authorized Shares Issued and Outstanding Net Carrying Value Aggregate Liquidation Preference (In thousands, except share amounts) Series A 7,557,990 7,557,990 $ 264,365 $ 265,000 Series A-1 4,185,997 4,185,997 146,687 146,770 11,743,987 11,743,987 $ 411,052 $ 411,770 The Company classified the convertible preferred stock outside of stockholders’ deficit because, in the event of certain “liquidation events” that are not solely within the control of the Company (including merger, acquisition, or sale of all or substantially all of the assets), the shares would become redeemable at the option of the holders. The Company did not adjust the carrying values of the convertible preferred stock to the deemed liquidation values of such shares since a liquidation event was not probable at the reporting date. 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. Common Stock Pursuant to the Amended and Restated Certificate of Incorporation filed on April 5, 2018, as amended, the Company was authorized to issue a total of 101,000,000 shares of common stock, of which 31,270,573 shares were issued and outstanding at September 30, 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. For the three and nine months ended September 30, 2018, the Company recognized $3.4 million and $11.4 million of this amount in general and administrative expense in the accompanying condensed statements of operations. 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 September 30, 2018, no dividends on common stock had been declared by the Company’s board of directors.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	1 1 . Stock-Based Compensation 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. Under the terms of the 2018 Plan, options may be granted at an exercise price not less than fair market value. 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. As of September 30, 2018, there were 1,112,753 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: Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contract Term (In years) Balance, December 31, 2017 — $ — Options granted 11,156,250 $ 3.23 Options exercised (5,020,580 ) $ 2.26 Options forfeited (59,851 ) $ 2.26 Balance, September 30, 2018 6,075,819 $ 4.04 9.81 Exercisable, September 30, 2018 2,657,550 $ 3.94 9.81 Vested and expected to vest, September 30, 2018 6,075,819 $ 4.04 9.81 Total stock-based compensation related to stock options and modification of the founders’ common stock (see Note 10) was as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 (In thousands) Research and development $ 427 $ 442 General and administrative 4,212 12,253 Total stock-based compensation $ 4,639 $ 12,695 Early Exercised Options

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions As of September 30, 2018, Pfizer held 4,185,997 shares of Series A-1 convertible preferred stock and had appointed two members 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 three and nine months ended September 30, 2018, the costs incurred under the Pfizer TSA were $3.8 million and $7.4 million, respectively. The costs for the three and nine months ended September 30, 2018, were recorded in general and administrative expense in the amounts of $1.9 million and $3.7 million, respectively, and in research and development expense in the amounts $1.9 million and $3.7 million, respectively. The Company also purchased certain lab supplies from Pfizer in connection with its research and development activities. For the three and nine months ended September 30, 2018, the total lab supplies and services purchased from Pfizer was $2.8 million and $6.1 million, respectively, which were recorded as research and development expense. As of September 30, 2018, the Company had an amount payable to Pfizer of $4.4 million, which was recorded in the accrued and other current liabilities on the accompanying condensed balance sheets. Consulting Agreements In June 2018, the Company entered into a services agreement with 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. Additionally, in June 2018 the Company entered into a consulting services agreement with a firm affiliated with a beneficial owner of more than 5% of the Company’s capital stock. The costs incurred for services provided under these agreements were $0.3 million and $0.7 million for the three and nine months ended September 30, 2018, respectively, and were 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, in 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 and out-of-pocket expenses incurred under this consulting agreement were $0.1 million and $0.1 million for the three and nine months ended September 30, 2018 and were included in general and administrative expenses.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	13. In December 2017, the Tax Cuts and Jobs Act (Tax Act) was signed into law. The Tax Act, among other changes, lowers the Company’s federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities as of December 31, 2017 to reflect the lower statutory tax rate. However, since the Company established a valuation allowance to offset its deferred tax assets, there was no impact to the effective tax rate, as any changes to deferred taxes would be offset by the valuation allowance. The deferred tax remeasurement is provisional and is subject to revision as the Company completes its analysis of the Tax Act, collects and prepares necessary data and interprets any additional guidance issued by standard-setting bodies. The Company currently anticipates finalizing and recording any resulting adjustments related to the tax effects of the Tax Act in 2018.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	14. Net Loss Per Share The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the period presented due to their anti-dilutive effect: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 (In thousands) Convertible preferred stock (on an as-converted basis) 61,655,922 61,655,922 Convertible notes payable (on an as-converted basis) 7,856,176 7,856,176 Stock options to purchase common stock 6,075,819 6,075,819 Founder shares of common stock subject to future vesting 21,201,908 21,201,908 Early exercised stock options subject to future vesting 5,020,575 5,020,575 Total 101,810,400 101,810,400

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Event	1 5 . Subsequent Event In October 2018, the Company entered into an operating lease agreement for office and laboratory space in South San Francisco, California near the Company headquarters. The aggregate lease payment amounts over the lease term are approximately $10.8 million. In connection with the lease, the Company will maintain a letter of credit for the benefit of the landlord in the amount of $0.2 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) for interim financial information and pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the Company’s opinion, all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation of the results of operations and cash flows for the periods presented have been included. The condensed balance sheet as of September 30, 2018, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2018, the condensed statement of cash flows for the nine months ended September 30, 2018, and the financial data and other financial information disclosed in the notes to the condensed financial statements are unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. These condensed financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated October 10, 2018 that forms a part of the Company's Registration Statements on Form S-1 (File Nos. 333-227333 and 333-227774), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended, on October 11, 2018.
Use of Estimates	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 convertible notes payable, 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 dictate. Actual results could differ from those estimates.
Cash, Cash Equivalents and Restricted Cash	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 a letter of credit under a lease agreement which has been collateralized by restricted cash. This cash is classified as long-term restricted cash on the accompanying condensed balance sheet based on the term of the underlying lease.
Investments	Investments Investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. 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.
Fair Value Measurement	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.
Deferred Offering Costs	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. As of September 30, 2018, $2.2 million of deferred offering costs have been capitalized, which is included in other long-term assets in the accompanying condensed balance sheet. There were no deferred offering costs recorded as of December 31, 2017.
Leases	Leases The Company early adopted Accounting Standards Update (ASU) No. 2016-02, Leases as of January 1, 2018 in accordance with ASC 250, Accounting Changes and Error Corrections For its long-term operating 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 nonlease components for its long-term real estate leases.
Accrued Research and Development Costs	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 its condensed balance sheets and within research and development expenses on the statements of operations.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses for the nine months ended September 30, 2018 primarily consist of acquired intangible assets as research and development costs pursuant to the Asset Contribution Agreement with Pfizer (see Note 7) 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 nine months ended September 30, 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.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In March 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2016-09, Stock Compensation—Improvements to Employee Share-Based Payment Accounting In January 2017, the FASB issued Accounting Standards Update, Business Combinations (Topic 805): Clarifying the Definition of a Business In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In November 2016, the FASB issued Accounting Standards Update No. Statement of Cash Flows: Restricted Cash 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. . The amendments in this ASU should be applied retrospectively to all periods presented. The Company The adoption of this ASU Leases The Company early adopted Accounting Standards Update (ASU) No. 2016-02, Leases as of January 1, 2018 in accordance with ASC 250, Accounting Changes and Error Corrections commenced on August 1, 2018.
Recent Accounting Pronouncements Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In February 2018, the FASB issued Accounting Standards Update No. Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In August 2018, the FASB issued Accounting Standards Update No. Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Financial Assets And Liabilities Subject To Fair Value Measurements On Recurring Basis And Level Of Inputs Used In Such Measurements By Major Security Type	September 30, 2018 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 6,524 $ — $ — $ 6,524 Corporate bonds — 163,319 — 163,319 U.S. treasury securities 224,947 — — 224,947 Total financial assets $ 231,471 $ 163,319 $ — $ 394,790 Financial Liabilities: Convertible notes payable $ — $ — $ 139,615 $ 139,615 Total financial liabilities $ — $ — $ 139,615 $ 139,615

Financial Instruments (Tables)

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2018 are presented in the following table: September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 6,524 $ — $ — $ 6,524 Corporate bonds 163,413 18 (112 ) 163,319 U.S. treasury securities 225,001 — (54 ) 224,947 Total cash equivalents, restricted cash and investments $ 394,938 $ 18 $ (166 ) $ 394,790 Classified as: Cash equivalents $ 78,728 Restricted cash 937 Short-term investments 161,080 Long-term investments 154,045 Total cash equivalents, restricted cash and investments $ 394,790

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Intangible Assets, Net	Intangible Assets, Net September 30, 2018 Cost Accumulated Amortization Carrying Value (in thousands) Assembled workforce $ 1,206 $ (302 ) $ 904
Schedule of Accrued Liabilities	Accrued Liabilities Accrued liabilities consist of the following: September 30, 2018 December 31, 2017 (In thousands) Accrued research and development expenses $ 6,584 $ — Unvested shares liabilities 3,883 — Accrued compensation and related benefits 1,728 — Accrued offering costs 579 — Accrued professional and consulting services 2,066 — Other 1,939 2 Total accrued and other current liabilities $ 16,779 $ 2

Asset Acquisition (Tables)

Asset Acquisition (Tables)	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Schedule of Fair Value of Assets Acquired	The following table summarizes the fair value of assets acquired (in thousands): Property and equipment $ 3,258 In-process research and development (IPR&D): Anti-CD19 CAR T cell therapy 103,936 Anti-BCMA CAR T cell therapy 5,500 Assembled workforce 1,206 Total assets acquired $ 113,900

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Undiscounted Future Lease Payments Under Lease Liability	The undiscounted future lease payments under the lease liability as of September 30, 2018 were as follows: Year ending December 31: (in thousands) 2018 (remaining three months) $ — 2019 2,406 2020 4,245 2021 4,394 2022 4,548 2023 and thereafter 31,769 Total undiscounted lease payments 47,362 Less: Present value adjustment (16,596 ) Less: Tenant improvement allowance (5,105 ) Total $ 25,661

Convertible Notes Payable (20_2

Convertible Notes Payable (2018 Notes) (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Summary on Key Valuation Assumptions	The Company determined the fair value of the 2018 Notes as of September 30, 2018 by determining the present value of the 2018 Notes if they were to settle either upon the closing of an IPO or in shares of Series B convertible preferred stock one year after issuance (September 2019). Key valuation assumptions are as follows: As of September 30, 2018 IPO Scenario Series B Scenario Expected term (years) 0.03 – 0.10 1 year Discount rate 20% — Weighting 95% 5%

Convertible Preferred Stock a_2

Convertible Preferred Stock and Stockholders’ Deficit (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Summary of Convertible Preferred Stock	Convertible preferred stock consisted of the following: September 30, 2018 Shares Authorized Shares Issued and Outstanding Net Carrying Value Aggregate Liquidation Preference (In thousands, except share amounts) Series A 7,557,990 7,557,990 $ 264,365 $ 265,000 Series A-1 4,185,997 4,185,997 146,687 146,770 11,743,987 11,743,987 $ 411,052 $ 411,770

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 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: Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contract Term (In years) Balance, December 31, 2017 — $ — Options granted 11,156,250 $ 3.23 Options exercised (5,020,580 ) $ 2.26 Options forfeited (59,851 ) $ 2.26 Balance, September 30, 2018 6,075,819 $ 4.04 9.81 Exercisable, September 30, 2018 2,657,550 $ 3.94 9.81 Vested and expected to vest, September 30, 2018 6,075,819 $ 4.04 9.81
Schedule of Stock-Based Compensation Expense	Total stock-based compensation related to stock options and modification of the founders’ common stock (see Note 10) was as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 (In thousands) Research and development $ 427 $ 442 General and administrative 4,212 12,253 Total stock-based compensation $ 4,639 $ 12,695

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the period presented due to their anti-dilutive effect: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 (In thousands) Convertible preferred stock (on an as-converted basis) 61,655,922 61,655,922 Convertible notes payable (on an as-converted basis) 7,856,176 7,856,176 Stock options to purchase common stock 6,075,819 6,075,819 Founder shares of common stock subject to future vesting 21,201,908 21,201,908 Early exercised stock options subject to future vesting 5,020,575 5,020,575 Total 101,810,400 101,810,400

Description of Business - Addit

Description of Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Oct. 10, 2018USD ($)$ / sharesshares	Oct. 01, 2018	Dec. 31, 2017USD ($)shares	Sep. 30, 2018USD ($)shares	Sep. 30, 2018USD ($)shares
Subsidiary Sale Of Stock [Line Items]
Date of incorporation					Nov. 30, 2017
Start up costs | $			$ 2,000
Convertible preferred stock, shares outstanding			0	11,743,987	11,743,987
Cash and cash equivalents and marketable securities | $				$ 398,300	$ 398,300
Cumulative net losses | $				$ (43,497)	(180,996)
IPO
Subsidiary Sale Of Stock [Line Items]
Offering costs | $					$ 2,200
Number of 2018 Notes converted into common stock					7,856,176
Subsequent Event
Subsidiary Sale Of Stock [Line Items]
Net proceeds | $	$ 343,000
Common stock forward split ratio		0.1905
Subsequent Event | IPO
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares	20,700,000
Common stock price per share | $ / shares	$ 18
Underwriting discounts and commissions | $	$ 26,100
Offering costs | $	$ 3,500
Convertible preferred stock, shares outstanding	11,743,987
Preferred stock outstanding	0
Subsequent Event | Over-Allotment Option
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock, shares	2,700,000
Common Stock | Subsequent Event | IPO
Subsidiary Sale Of Stock [Line Items]
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

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Deferred offering costs	$ 2,200	$ 0
Expense related to acquired intangible in-process research and development recognized	$ 109,436

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring $ in Thousands	Sep. 30, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	$ 394,790
Financial Liabilities	139,615
Convertible Notes Payable
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Liabilities	139,615
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	6,524
Corporate Bond Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	163,319
US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	224,947
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	231,471
Financial Liabilities	0
Level 1 | Convertible Notes Payable
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Liabilities	0
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	6,524
Level 1 | Corporate Bond Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Level 1 | US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	224,947
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	163,319
Financial Liabilities	0
Level 2 | Convertible Notes Payable
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Liabilities	0
Level 2 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Level 2 | Corporate Bond Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	163,319
Level 2 | US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Financial Liabilities	139,615
Level 3 | Convertible Notes Payable
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Liabilities	139,615
Level 3 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Level 3 | Corporate Bond Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	0
Level 3 | US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	$ 0

Financial Instruments - Summary

Financial Instruments - Summary of Cash Equivalents, Restricted Cash and Investments, Classified as Available-for-Sale Securities (Details) $ in Thousands	Sep. 30, 2018USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 394,938
Unrealized Gains	18
Unrealized Losses	(166)
Fair Value	394,790
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	6,524
Unrealized Gains	0
Unrealized Losses	0
Fair Value	6,524
Corporate Bond Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	163,413
Unrealized Gains	18
Unrealized Losses	(112)
Fair Value	163,319
US Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	225,001
Unrealized Gains	0
Unrealized Losses	(54)
Fair Value	224,947
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Fair Value	78,728
Restricted Cash
Schedule Of Available For Sale Securities [Line Items]
Fair Value	937
Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Fair Value	161,080
Long-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Fair Value	$ 154,045

Financial Instruments - Additio

Financial Instruments - Additional Information (Details)	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Maximum remaining contractual maturities of available-for-sale securities	2 years

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Intangible Assets (Details) - Assembled Workforce $ in Thousands	Sep. 30, 2018USD ($)
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Intangible Assets, Cost	$ 1,206
Intangible Assets, Accumulated Amortization	(302)
Intangible Assets, Carrying Value	$ 904

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Balance Sheet Components [Line Items]
Intangible assets, weighted-average remaining amortization period		1 year 6 months 3 days
Amortization expense		$ 302
Assembled Workforce
Balance Sheet Components [Line Items]
Amortization expense	$ 200	$ 300

Balance Sheet Components - Sc_2

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Accrued research and development expenses	$ 6,584
Unvested shares liabilities	3,883
Accrued compensation and related benefits	1,728
Accrued offering costs	579
Accrued professional and consulting services	2,066
Other	1,939	$ 2
Total accrued and other current liabilities	$ 16,779	$ 2	[1]
[1]	The balance sheet as of December 31, 2017 is derived from the audited financial statements as of that date.

Asset Acquisition - Additional

Asset Acquisition - Additional Information (Details) $ / shares in Units, $ in Millions	1 Months Ended
	Apr. 30, 2018USD ($)$ / sharesshares
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
Aggregate milestone payments	$ 840
Pfizer | Asset Contribution Agreement | UNITED STATES
Asset Acquisition [Line Items]
Milestone payments, per target	30
Pfizer | Asset Contribution Agreement | European Union
Asset Acquisition [Line Items]
Milestone payments, per target	$ 60
Pfizer | Asset Contribution Agreement | Series A1 Convertible Preferred Shares
Asset Acquisition [Line Items]
Share issued in connection with asset acquisition | shares	3,187,772
Share issued in connection with asset acquisition, fair value	$ 111.8
Convertible preferred stock, price per share | $ / shares	$ 35.06
Costs from issuance of preferred shares	$ 2.1
Total asset acquisition	$ 113.9

Asset Acquisition - Schedule of

Asset Acquisition - Schedule of Fair Value of Assets Acquired (Details) $ in Thousands	Apr. 30, 2018USD ($)
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 Agreements - Additional

License Agreements - Additional Information (Details) € in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2018EUR (€)	Dec. 31, 2017USD ($)	[1]
License Agreements [Line Items]
Research and development	$ 10,870,000	$ 133,356,000
Accrued and other current liabilities	16,779,000	16,779,000		$ 2,000
Accrued and other current liabilities	4,600,000	$ 4,600,000
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 | European Union | Maximum
License Agreements [Line Items]
Aggregate potential regulatory and development milestones		840,000,000
Pfizer | Asset Contribution Agreement | North America, Europe, Asia, Australia and Oceania
License Agreements [Line Items]
Aggregate potential milestone payments		325,000,000
Cellectis | Research Collaboration and License Agreement
License Agreements [Line Items]
Maximum payments required per product against selected target		185,000,000
Research and development	0	400,000
Accrued and other current liabilities	0	0
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	4,800,000	$ 7,500,000
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	$ 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	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
[1]	The balance sheet as of December 31, 2017 is derived from the audited financial statements as of that date.

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2018USD ($)Sq.Ft	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)
Operating Expenses [Abstract]
Area of office and laboratory | Sq.Ft	68,000
Operating lease term	127 months
Operating lease expiration period	Feb. 28, 2029
Operating lease, option to extend term	7 years
Allowance for tenant improvements	$ 5,100	$ 5,105	$ 5,105
Letter of credit	$ 900
Operating lease right-of-use asset		24,996	24,996
Lease liability, noncurrent		25,661	25,661
Remaining Lease term	10 years 4 months 24 days
Lease discount rate	8.00%
Rent expense		700	700
Short term lease expense		$ 1,000	$ 1,700

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Undiscounted Future Lease Payments Under Lease Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Aug. 31, 2018
Operating Leases Future Minimum Payments Due [Abstract]
2,019	$ 2,406
2,020	4,245
2,021	4,394
2,022	4,548
2023 and thereafter	31,769
Total undiscounted lease payments	47,362
Less: Present value adjustment	(16,596)
Less: Tenant improvement allowance	(5,105)	$ (5,100)
Total	$ 25,661

Convertible Notes Payable (20_3

Convertible Notes Payable (2018 Notes) - Additional information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 05, 2018
Short Term Debt [Line Items]
Aggregate principal amount	$ 120,200,000	$ 120,200,000
Proceeds from issuance of convertible notes, net of issuance costs		116,842,000
Fair value of notes	139,600	139,600	$ 120,200
Change in fair value of convertible notes payable	19,400	19,400
Debt issuance of cost	3,400,000	3,400,000
IPO
Short Term Debt [Line Items]
Fully diluted capitalization value upon immediate conversion of notes	$ 900,000,000	$ 900,000,000
Number of 2018 Notes converted into common stock		7,856,176

Convertible Notes Payable (20_4

Convertible Notes Payable (2018 Notes) - Summary on Key Valuation Assumptions (Details)	9 Months Ended
	Sep. 30, 2018
Series B Preferred Stock
Short Term Debt [Line Items]
Weighting	5.00%
Maximum | Series B Preferred Stock
Short Term Debt [Line Items]
Expected term (years)	1 year
IPO
Short Term Debt [Line Items]
Discount rate	20.00%
Weighting	95.00%
IPO | Maximum
Short Term Debt [Line Items]
Expected term (years)	11 days
IPO | Minimum [Member]
Short Term Debt [Line Items]
Expected term (years)	1 month 6 days

Convertible Preferred Stock a_3

Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Oct. 10, 2018	Aug. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		11,743,987	11,743,987			0
Proceeds from issuance of convertible preferred stock net			$ 299,282
Aggregate purchase price, shares value	$ 300,000
Subscriptions receivable					$ 150,000
Common stock, shares authorized		101,000,000	101,000,000			47,250,000
Common stock, shares issued		31,270,573	31,270,573			26,249,993
Common stock, shares outstanding		31,270,573	31,270,573			26,249,993
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
General and administrative
Class Of Stock [Line Items]
Compensation cost resulting from the modification		$ 3,400	$ 11,400
Subsequent Event | IPO
Class Of Stock [Line Items]
Share price				$ 18
Subsequent Event | Common Stock | IPO
Class Of Stock [Line Items]
Number of convertible preferred stock converted to shares of common stock				61,655,922
Series A-1 Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible preferred stock, shares issued	998,225	4,185,997	4,185,997
Share price	$ 35.06
Proceeds from issuance of convertible preferred stock net	$ 34,900
Series A-1 Convertible Preferred Shares | Pfizer
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		4,185,997	4,185,997
Series A-1 Convertible Preferred Shares | Pfizer | Asset Contribution Agreement
Class Of Stock [Line Items]
Convertible preferred stock, shares issued	3,187,772
Series A Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible preferred stock, shares issued	7,557,990	7,557,990	7,557,990
Share price	$ 35.06
Proceeds from issuance of convertible preferred stock net	$ 264,400

Convertible Preferred Stock a_4

Convertible Preferred Stock and Stockholders' Deficit - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Apr. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized	11,743,987		1,000,000
Shares Issued	11,743,987		0
Shares Outstanding	11,743,987		0
Net Carrying Value	$ 411,052
Convertible preferred stock, aggregate liquidation Preference	$ 411,770
Series A Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized	7,557,990
Shares Issued	7,557,990	7,557,990
Shares Outstanding	7,557,990
Net Carrying Value	$ 264,365
Convertible preferred stock, aggregate liquidation Preference	$ 265,000
Series A-1 Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized	4,185,997
Shares Issued	4,185,997	998,225
Shares Outstanding	4,185,997
Net Carrying Value	$ 146,687
Convertible preferred stock, aggregate liquidation Preference	$ 146,770

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) $ in Millions	Sep. 30, 2018USD ($)shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Accrued and other liabilities, related to shares held by employees and directors that were subject to repurchase	$ 3.9
Other long term liabilities, related to shares held by employees and directors that were subject to repurchase	$ 7.5
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance | shares	1,112,753

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity Under Plan (Details) - 2018 Plan	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Shares
Number of Options, Balance | shares	0
Number of Options, Options granted | shares	11,156,250
Number of Options, Options exercised | shares	(5,020,580)
Number of Options, Options forfeited | shares	(59,851)
Number of Options, Balance | shares	6,075,819
Number of Options, Exercisable | shares	2,657,550
Number of Options, Vested and expected to vest | shares	6,075,819
Weighted-Average Exercise Price, Balance | $ / shares	$ 0
Weighted-Average Exercise Price, Options granted | $ / shares	3.23
Weighted-Average Exercise Price, Options exercised | $ / shares	2.26
Weighted-Average Exercise Price, Options forfeited | $ / shares	2.26
Weighted-Average Exercise Price, Balance | $ / shares	4.04
Weighted-Average Exercise Price, Exercisable | $ / shares	3.94
Weighted-Average Exercise Price, Vested and expected to vest | $ / shares	$ 4.04
Weighted-Average Remaining Contract Term, Balance	9 years 9 months 21 days
Weighted-Average Remaining Contract Term, Exercisable	9 years 9 months 21 days
Weighted-Average Remaining Contract Term, Vested and expected to vest	9 years 9 months 21 days

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 4,639	$ 12,695
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	427	442
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 4,212	$ 12,253

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2018USD ($)	Sep. 30, 2018USD ($)Directorshares	Sep. 30, 2018USD ($)Directorshares	Jun. 30, 2018	Apr. 30, 2018shares	Dec. 31, 2017shares
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued | shares		11,743,987	11,743,987			0
Affiliated with President and Chief Executive Officer
Related Party Transaction [Line Items]
Ownership percentage				5.00%
General and administrative | Affiliated with President and Chief Executive Officer
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		$ 300	$ 700
Series A1 Convertible Preferred Shares
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued | shares		4,185,997	4,185,997		998,225
Pfizer
Related Party Transaction [Line Items]
Number of members appointed to board of directors | Director		2	2
Pfizer | Accrued and Other Current Liabilities
Related Party Transaction [Line Items]
Payable to related party, current		$ 4,400	$ 4,400
Pfizer | Transition Services Agreement
Related Party Transaction [Line Items]
Related party costs		3,800	7,400
Related party general and administrative expense		1,900	3,700
Pfizer | Research and Development Expense | Transition Services Agreement
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		1,900	3,700
Pfizer | Research and Development Expense | Transition Services Agreement | Lab Supplies
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party		$ 2,800	$ 6,100
Pfizer | Series A1 Convertible Preferred Shares
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued | shares		4,185,997	4,185,997
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		$ 100	$ 100

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	34.00%

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	101,810,400	101,810,400
Convertible Preferred Stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	61,655,922	61,655,922
Convertible Notes Payable (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	7,856,176	7,856,176
Stock Options To Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation of diluted net loss per share	6,075,819	6,075,819
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	21,201,908	21,201,908
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	5,020,575	5,020,575

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Aug. 31, 2018
Subsequent Event [Line Items]
Operating lease term			127 months
Allowance for tenant improvements		$ 5,105	$ 5,100
Letter of credit			$ 900
Operating lease, option to extend term			7 years
Subsequent Event
Subsequent Event [Line Items]
Operating lease term	10 years 4 months
Allowance for tenant improvements	$ 800
Letter of credit	$ 200
Operating lease commencement date	Nov. 1, 2018
Operating lease, option to extend term	7 years
Aggregate lease payment amounts over lease term	$ 10,800