Lyell Immunopharma (LYEL) 10-Q 12 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 05, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Entity Central Index Key	0001806952
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity File Number	001-40502
Entity Registrant Name	Lyell Immunopharma, Inc.
Entity Incorporation State Country Code	DE
Entity Tax Identification Number	83-1300510
Entity Address Address Line1	201 Haskins Way
Entity Address City Or Town	South San Francisco
Entity Address State Or Province	CA
Entity Address Postal Zip Code	94080
City Area Code	650
Local Phone Number	695-0677
Security12b Title	Common Stock, $0.0001 par value
Trading Symbol	LYEL
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		240,471,497

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 393,504	$ 140,406
Short-term marketable securities	225,417	472,213
Prepaid expenses and other current assets	12,226	4,928
Total current assets	631,147	617,547
Restricted cash	466	466
Long-term marketable securities	317,511	79,995
Other investments	83,448	83,448
Property and equipment, net	115,857	77,045
Right-of-use assets, net	47,236	47,010
Other non-current assets	2,669	2,769
Total assets	1,198,334	908,280
Current liabilities:
Accounts payable	3,526	9,396
Accrued liabilities and other current liabilities	25,058	28,021
Success payment liabilities	25,116	5,773
Deferred revenue	5,581	6,095
Total current liabilities	59,281	49,285
Operating lease liabilities, non-current	65,684	50,957
Deferred revenue, non current	81,883	89,066
Other non-current liabilities	5,012	532
Total liabilities	211,860	189,840
Commitments and contingencies (Note 12)
Convertible preferred stock, $0.0001 par value; no and 195,021 shares authorized at September 30, 2021 and December 31, 2020, respectively; no and 194,474 shares were issued and outstanding at September 30, 2021 and at December 31, 2020, respectively	0	1,010,968
Stockholders' deficit:
Preferred stock: $0.0001 par value, 0 and no shares authorized at September 30, 2021 and December 31, 2020, respectively; no shares issued and outstanding at September 30, 2021 and December 31, 2020	0	0
Common stock, $0.0001 par value; 264,905 shares authorized at September 30, 2021 and December 31, 2020, respectively; 239,789 and 15,570 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively	24	2
Additional paid-in capital	1,487,171	41,357
Accumulated other comprehensive (loss) income	(65)	256
Accumulated deficit	(500,656)	(334,143)
Total stockholders' deficit	986,474	(292,528)
Total liabilities, convertible preferred stock and stockholders' deficit	$ 1,198,334	$ 908,280

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	195,021,000
Convertible preferred stock, shares issued	0	194,474,000
Outstanding convertible preferred stock (in shares)	0	194,474,000
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	264,905,000
Common stock, shares, issued	239,789,000	15,570,000
Common Stock, shares, outstanding	239,789,000	15,570,000

Condensed Consolidated Statemen

Condensed Consolidated Statement of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Revenue	$ 2,755	$ 1,070	$ 7,828	$ 5,444
Operating expenses (income):
Research and development	31,433	24,501	119,408	147,153
General and administrative	21,241	13,570	57,184	32,012
Other operating income, net	(758)	(158)	(1,526)	(1,308)
Total operating expenses	51,916	37,913	175,066	177,857
Loss from operations	(49,161)	(36,843)	(167,238)	(172,413)
Interest income	270	1,096	842	5,318
Other income (expense), net	16	28	(117)	1,480
Net loss	(48,875)	(35,719)	(166,513)	(165,615)
Other comprehensive (loss) gain:
Net unrealized gain (loss) on marketable securities	(138)	(598)	(321)	139
Net comprehensive loss	(49,013)	(36,317)	(166,834)	(165,476)
Net loss attributed to common stockholders:
Net loss	(48,875)	(35,719)	(166,513)	(165,615)
Deemed dividends upon repurchase of convertible preferred stock				(3,582)
Net loss attributed to common stockholders	$ (48,875)	$ (35,719)	$ (166,513)	$ (169,197)
Net loss per common share, basic and diluted	$ (0.20)	$ (2.57)	$ (1.66)	$ (13.40)
Weighted-average shares used to compute net loss per common share, basic and diluted	239,384	13,893	100,603	12,625

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2019	$ (111,108)		$ 1	$ 18,108	$ 454	$ (129,671)
Beginning balance, (in shares) at Dec. 31, 2019		152,116,000
Beginning balance at Dec. 31, 2019		$ 519,163
Beginning balance, (in shares) at Dec. 31, 2019			11,181,000
Issuance of Series C convertible preferred stock		$ 492,467
Issuance of Series C convertible preferred stock, shares		42,905,000
Issuance of common stock to strategic partners	1,004			1,004
Issuance of common stock to strategic partners, shares			275,000
Issuance of common stock for asset acquisition	4,000			4,000
Issuance of common stock for asset acquisition, shares			688,000
Repurchase of convertible preferred stock	(3,582)	$ (662)		(3,582)
Repurchase of convertible preferred stock, shares		(547,000)
Issuance of common stock upon exercise of stock options	35			35
Issuance of common stock upon exercise of stock options, shares			15,000
Repurchase of common stock	(11,806)			(11,806)
Repurchase of common stock, shares			(2,032,000)
Stock-based compensation	20,290			20,290
Stock-based compensation, shares			4,177,000
Other comprehensive income	139				139
Net loss	(165,615)					(165,615)
Ending balance at Sep. 30, 2020	(266,643)		$ 1	28,049	593	(295,286)
Ending balance, (in shares) at Sep. 30, 2020		194,474,000
Ending balance at Sep. 30, 2020		$ 1,010,968
Ending balance, (in shares) at Sep. 30, 2020			14,304,000
Beginning balance at Jun. 30, 2020	(240,109)		$ 1	18,266	1,191	(259,567)
Beginning balance, (in shares) at Jun. 30, 2020		194,474,000
Beginning balance at Jun. 30, 2020		$ 1,010,968
Beginning balance, (in shares) at Jun. 30, 2020			13,097,000
Stock-based compensation	9,783			9,783
Stock-based compensation, shares			1,207,000
Other comprehensive income	(598)				(598)
Net loss	(35,719)					(35,719)
Ending balance at Sep. 30, 2020	(266,643)		$ 1	28,049	593	(295,286)
Ending balance, (in shares) at Sep. 30, 2020		194,474,000
Ending balance at Sep. 30, 2020		$ 1,010,968
Ending balance, (in shares) at Sep. 30, 2020			14,304,000
Beginning balance at Dec. 31, 2020	$ (292,528)		$ 2	41,357	256	(334,143)
Beginning balance, (in shares) at Dec. 31, 2020	194,474,000	194,474,000
Beginning balance at Dec. 31, 2020	$ 1,010,968	$ 1,010,968
Beginning balance, (in shares) at Dec. 31, 2020	15,570,000		15,570,000
Proceeds from initial public offering, net of issuance costs, Shares			25,000,000
Proceeds from initial public offering, net of issuance costs	$ 391,802		$ 2	391,800
Conversion of convertible preferred stock to common stock, Shares		194,474,000	194,474,000
Conversion of convertible preferred stock to common stock	1,010,968	$ (1,010,968)	$ 20	1,010,948
Issuance of common stock upon exercise of stock options	$ 2,043			2,043
Issuance of common stock upon exercise of stock options, shares	776,449		776,000
Stock-based compensation	$ 41,023			41,023
Stock-based compensation, shares			3,969,000
Other comprehensive income	(321)				(321)
Net loss	(166,513)					(166,513)
Ending balance at Sep. 30, 2021	$ 986,474		$ 24	1,487,171	(65)	(500,656)
Ending balance, (in shares) at Sep. 30, 2021	0
Ending balance at Sep. 30, 2021	$ 0
Ending balance, (in shares) at Sep. 30, 2021	239,789,000		239,789,000
Beginning balance at Jun. 30, 2021	$ 1,021,955		$ 24	1,473,639	73	(451,781)
Beginning balance, (in shares) at Jun. 30, 2021			238,549,000
Initial public offering costs	(39)			(39)
Issuance of common stock upon exercise of stock options	529			529
Issuance of common stock upon exercise of stock options, shares			265,000
Stock-based compensation	13,042			13,042
Stock-based compensation, shares			975,000
Other comprehensive income	(138)				(138)
Net loss	(48,875)					(48,875)
Ending balance at Sep. 30, 2021	$ 986,474		$ 24	$ 1,487,171	$ (65)	$ (500,656)
Ending balance, (in shares) at Sep. 30, 2021	0
Ending balance at Sep. 30, 2021	$ 0
Ending balance, (in shares) at Sep. 30, 2021	239,789,000		239,789,000

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (Unaudited) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
Convertible Preferred Stock
Net issuance cost	$ 533

Condensed Consolidated Statem_4

Condensed Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (166,513)	$ (165,615)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	9,059	2,842
Stock-based compensation expense	41,023	20,290
Change in fair value of success payment liabilities	19,343	3,793
Change in fair value of warrants	199	(1,295)
Non-cash lease expense	1,486	4,694
Non-cash expense in connection with asset acquisition	0	3,529
Net amortization or accretion on marketable securities	1,434	67
Loss on property and equipment disposals	496	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(6,811)	(596)
Accounts payable	(8)	(616)
Accrued liabilities and other current liabilities	10,792	4,209
Deferred revenue	(7,697)	(5,444)
Other non-current liabilities	4,121	0
Net cash used in operating activities	(93,076)	(134,142)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(55,236)	(26,600)
Sales of property and equipment	40
Purchases of marketable securities	(539,541)	(723,487)
Sales and maturities of marketable securities	547,066	480,631
Purchases of other investments		(36,447)
Net cash used in investing activities	(47,671)	(305,903)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from initial public offering, net of issuance costs	391,802
Proceeds from issuance of convertible preferred stock, net of issuance costs		492,467
Proceeds from exercise of stock options	2,043	35
Payments for the repurchase of common stock		(11,806)
Payments for the repurchase of preferred stock		(4,244)
Net cash provided by financing activities	393,845	476,452
Net increase in cash, cash equivalents and restricted cash	253,098	36,407
Cash, cash equivalents and restricted cash at beginning of period	140,872	98,472
Cash, cash equivalents and restricted cash at end of period	393,970	134,879
Represented by:
Cash and cash equivalents	393,504	134,413
Restricted cash	466	466
SUPPLEMENTAL CASH FLOW INFORMATION
Conversion of convertible preferred stock to common stock upon closing of initial public offering	1,010,968
Purchases of property and equipment included in accounts payable and accrued liabilities	5,432	18,439
Operating lease right-of-use assets obtained in exchange for lease obligations		30,475
Remeasurement of operating lease right of use asset for lease modification	4,078	2,963
Cash received for amounts related to tenant improvement allowances	11,063	2,705
Cash paid for amounts included in the measurement of lease liabilities	$ 5,658	$ 3,628

Organization

Organization	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Organization	1. Organization Lyell Immunopharma, Inc. (the “Company”) was incorporated in Delaware in June 2018. The Company is a T cell reprogramming company dedicated to the mastery of T cells to cure patients with solid tumors. The Company is building a multi-modality product pipeline. The Company’s primary activities since incorporation have been to develop T cell therapies, perform research and development, acquire technology, enter into strategic collaboration and license arrangements, enable manufacturing activities in support of its product candidate development efforts, organize and staff the Company, conduct business planning, establish its intellectual property portfolio, raise capital and provide general and administrative support for these activities. Initial Public Offering In June 2021, the Company successfully completed its initial public offering (“IPO”) of its common stock. In connection with its IPO, the Company issued and sold 25,000,000 shares of common stock at an IPO price of $ 17.00 per share. The Company received $ 391.8 million in net proceeds, after deducting underwriting discounts and commissions of $ 29.8 million and offering expenses of $ 3.4 million. Upon the closing of the IPO, 194,474,431 shares of convertible preferred stock then outstanding converted into an equivalent number of shares of common stock. The related carrying value of the converted preferred stock of $ 1.0 billion was reclassified to common stock and additional paid in-capital.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation and Significant Accounting Policies	2. Basis of Presentation and Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The condensed consolidated financial statements include the accounts of Lyell Immunopharma, Inc. and its wholly-owned subsidiary. All significant intercompany transactions and balances are eliminated in consolidation. The condensed consolidated balance sheet as of December 31, 2020 included herein was derived from the audited consolidated financial statements as of that date. Certain information and footnote disclosures typically included in the Company's audited consolidated financial statements have been condensed or omitted. The accompanying unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s financial position, results of operations and cash flows for the periods presented, but are not necessarily indicative of results to be expected for any future annual or interim period. These condensed consolidated financial statements should be read in conjunction with the Company's audited financial statements and notes included in the prospectus the Company filed with the Securities and Exchange Commission on June 21, 2021 pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the "Prospectus"). Use of Estimates The preparation of the Company’s condensed consolidated financial statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect reported amounts and related disclosures. Specific accounts that require management estimates include, but are not limited to, stock-based compensation, valuation of success payments, revenue recognition and accrued expenses. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Actual results could differ materially from those estimates. Concentrations of Credit Risk and Off-Balance Sheet Risk The Company maintains its cash and cash equivalents and restricted cash with high quality, accredited financial institutions. These amounts, at times, may exceed federally insured limits. The Company also makes short-term investments in money market funds, U.S. Treasury securities, U.S. government agency securities and corporate debt securities, which can be subject to certain credit risk. However, the Company mitigates the risks by investing in high-grade instruments, limiting exposure to any one issuer or type of investment and monitoring the ongoing creditworthiness of the financial institutions and issuers. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to significant risk on these funds. The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements. Significant Accounting Policies There have been no significant changes to the accounting policies during the nine months ended September 30, 2021, as compared to the significant accounting policies described in Note 2 of the “Notes to Financial Statements” in the Company’s audited financial statements included in the Prospectus, with the exception of revenue recognition related to licenses of intellectual property during the nine months ended September 30, 2021. Revenue The Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements within the scope of Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers , ("ASC 606") the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the performance obligation is satisfied. In applying the ASC 606 framework, the Company must apply judgment to determine the nature of the promises within a revenue contract and whether those promises represent distinct performance obligations. In determining the transaction price, the Company does not include amounts subject to uncertainties unless it is probable that there will be no significant reversal of cumulative revenue when the uncertainty is resolved. Milestone and other forms of variable consideration that the Company may earn are subject to significant uncertainties of research and development related achievements, which generally are deemed not probable until such milestones are actually achieved. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). Additionally, the Company develops assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. The Company then allocates the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation, for which it recognizes revenue as or when the performance obligations are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the variable consideration and any related constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis. Under the Company’s license agreements, the Company grants the license to a customer as it exists at the point of transfer and the nature of the license is a right to use the Company’s intellectual property as transferred. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time.

Collaboration, License and Succ

Collaboration, License and Success Payment Agreements	9 Months Ended
	Sep. 30, 2021
Collaboration License And Success Payment Agreements [Abstract]
Collaboration License and Success Payment Agreements	3. Collaboration, License and Success Payment Agreements Fred Hutch In 2018, the Company entered into a license agreement with Fred Hutchinson Cancer Research Center (“Fred Hutch”) pertaining to certain patent rights. In 2018, the Company also entered into a research and collaboration agreement (“Fred Hutch Collaboration Agreement”), focused on research and development of cellular immunotherapy products and the Company recognized $ 1.1 million and $ 1.0 million of research and development expenses in connection with the Fred Hutch Collaboration Agreement for the three months ended September 30, 2021 and 2020, respectively, and $ 3.1 million and $ 3.0 million for the nine months ended September 30, 2021 and 2020, respectively. In 2018, the Company also granted Fred Hutch rights to certain success payments. The potential payments for the Fred Hutch success payments are based on multiples of increased value ranging from 10 x to 50 x based on a comparison of the estimated per share fair value of the Series A convertible preferred stock, or any security into which such stock has been converted or for which it has been exchanged, relative to its original $ 1.83 per share issuance price. Upon the closing of the IPO, all shares of Series A convertible preferred stock then outstanding converted into an equivalent number of shares of common stock. The aggregate success payments to Fred Hutch are not to exceed $ 200.0 million, which would only occur upon a 50 times increase in value. Each threshold is associated with a success payment, ascending from $ 10.0 million at $ 18.29 per share to $ 200.0 million at $ 91.44 per share, payable if such threshold is reached during the measurement period. Any previous success payments made are credited against the success payment owed as of any valuation date, such that Fred Hutch does not receive multiple success payments in connection with the same threshold. The term of the success payment agreement ends on the earlier to occur of (i) the nine year anniversary of the date of the agreement and (ii) a change in control transaction. The following table summarizes the aggregate potential success payments, which are payable to Fred Hutch in cash or cash equivalents or, at the Company’s discretion, publicly-tradeable shares of the Company’s common stock: Multiple of initial equity value at issuance 10 x 20 x 30 x 40 x 50 x Per share common stock price required for payment $ 18.29 $ 36.58 $ 54.86 $ 73.15 $ 91.44 Aggregate success payment(s) (in millions) $ 10 $ 40 $ 90 $ 140 $ 200 The success payments will be owed if the per share fair value of the Company's common stock on the contractually specified valuation measurement dates during the term of the success payment agreement equals or exceeds the above outlined multiples. The valuation measurement dates are triggered by the following events: the one-year anniversary of the Company's IPO and each two-year anniversary of the Company's IPO thereafter, the closing of a change in control transaction, and the last day of the term of the success payment agreement, unless the term has ended due to the closing of a change of control transaction. The estimated fair value of the success payments to Fred Hutch as of September 30, 2021 and December 31, 2020 was $ 21.7 million and $ 8.0 million, respectively. The success payment liability is estimated at fair value at inception and at each subsequent reporting period and the expense is accreted over the service period of the Fred Hutch Collaboration Agreement. With respect to Fred Hutch success payment obligations, the Company recognized expense of $( 0.8 ) million and $ 1.2 million for the three months ended September 30, 2021 and 2020, respectively, and $ 14.1 million and $ 3.8 million for the nine months ended September 30, 2021 and 2020, respectively. Expense associated with success payment obligations was recorded in research and development expense. Stanford In January 2019, the Company entered into a license agreement with The Board of Trustees of the Leland Stanford Junior University (“Stanford”) pertaining to certain patent rights. In October 2020, the Company entered into a research and collaboration agreement with Stanford (“Stanford Collaboration Agreement”), focused on research and development of cellular immunotherapy products. The Company recognized $ 0.8 million and $ 2.3 million of research and development expenses in connection with the Stanford Collaboration Agreement for the three and nine months ended September 30, 2021, respectively. As the Stanford Collaboration Agreement was entered into in October 2020, no expense was recognized for the three and nine months ended September 30, 2020. In October 2020, the Company also granted Stanford rights to certain success payments. The potential payments for the Stanford success payments are based on multiples of increased value ranging from 10x to 50x based on a comparison of the estimated per share fair value of the Series A convertible preferred stock, or any security into which such stock has been converted or for which it has been exchanged, relative to its original $ 1.83 per share issuance price. At the closing of the IPO, all shares of Series A convertible preferred stock then outstanding converted into an equivalent number of shares of common stock. The aggregate success payments to Stanford are not to exceed $ 200.0 million, which would only occur upon a 50 times increase in value. Each threshold is associated with a success payment, ascending from $ 10.0 million at $ 18.29 per share to $ 200.0 million at $ 91.44 per share, payable if such threshold is reached during the measurement period. Any previous success payments made are credited against the success payment owed as of any valuation date, so that Stanford does not receive multiple success payments in connection with the same threshold. The term of each success payment agreement ends on the earlier to occur of (i) the nine year anniversary of the date of the agreement and (ii) a change in control transaction. The following table summarizes the aggregate potential success payments, which are payable to Stanford in cash or cash equivalents or, at the Company’s discretion, publicly-tradeable shares of the Company’s common stock: Multiple of initial equity value at issuance 10 x 20 x 30 x 40 x 50 x Per share common stock price required for payment $ 18.29 $ 36.58 $ 54.86 $ 73.15 $ 91.44 Aggregate success payment(s) (in millions) $ 10 $ 40 $ 90 $ 140 $ 200 The success payments will be owed if the per share fair value of the Company's common stock on the contractually specified valuation measurement dates during the term of the success payment agreement equals or exceeds the above outlined multiples. The valuation measurement dates are triggered by the following events: the one-year anniversary of the Company's IPO and each two-year anniversary of the Company's IPO thereafter, the closing of a change in control transaction, and the last day of the term of the success payment agreement, unless the term has ended due to the closing of a change of control transaction. The estimated fair value of the success payments to Stanford as of September 30, 2021 and December 31, 2020 was $ 23.1 million and $ 8.9 million, respectively. The success payment liability is estimated at fair value at inception and at each subsequent reporting period and the expense is accreted over the service period of the Stanford Collaboration Agreement. With respect to Stanford success payment obligations, the Company recognized expense of $ 1.0 million for the three months ended September 30, 2021 and $ 5.2 million for the nine months ended September 30, 2021, which was recorded in research and development expense. As the rights to success payments were granted to Stanford in October 2020, no expense was recognized for the three and nine months ended September 30, 202 0 . GSK In 2019, the Company entered into a Collaboration and License Agreement, amended in June 2020 (“the GSK Agreement”) with GlaxoSmithKline Intellectual Property (No. 5) Limited and Glaxo Group Limited (together, “GSK”) for potential T cell therapies that apply the Company’s platform technologies and cell therapy innovations with T cell receptors (“TCRs”) or chimeric antigen receptors (“CARs”) under distinct collaboration programs. The GSK Agreement has defined two initial collaboration targets and allows GSK to nominate seven additional targets through July 2024. The Company is expected to perform research and development services for each selected target up until a defined point (the “GSK Option Point”), at which time GSK will decide whether or not to exercise an option to obtain a license from the Company (“License Option”) and take over the future development and commercialization. For each selected target, both parties will determine whether it will be developed under a Proof of Concept (“PoC”) Development Program or Component Development Program. For a PoC Development Program, the Company is expected to conduct both preclinical and clinical development for the target and present clinical trial data to GSK in connection with their evaluation of whether to exercise the License Option. For a Component Development Program, the Company is obligated to perform preclinical studies only. Along with the research activities, the Company appoints three representatives to the joint steering committee (“JSC”) and is responsible for the manufacture of all compounds and products necessary for its research and development activities. The Company received a non-refundable upfront payment of $ 45.0 million under the GSK Agreement. The Company is entitled to certain payments upon the achievement of specified development and sales milestones (for each selected target that is already within GSK’s pipeline and meets certain criteria, the Company is eligible to receive up to an aggregate of approximately $ 400.0 million, and for each selected target that is not already within GSK’s pipeline and meets certain criteria, the Company is eligible to receive up to an aggregate of approximately $ 900.0 million) and tiered royalties on a per-product basis ranging from low to high single digits for targets that are already within GSK’s pipeline and meet certain criteria, or from high single digit to low teens for all other targets. The Company is also entitled to potential milestone payments based on validating the Company’s technology in a clinical setting up to an aggregate of approximately $ 200.0 million. Royalties and milestones are paid once per target, even if there is more than one Lyell innovation applied to a T cell therapy directed to that target. Any amounts received from GSK are generally non-refundable unless the Company terminates a collaboration target for safety or feasibility reasons and the funding received from GSK exceeds the costs incurred for the terminated target. In connection with the GSK Agreement, in May 2019, the Company also entered into a Stock Purchase Agreement with GSK (the “GSK Stock Purchase Agreement”), pursuant to which the Company agreed to sell 30,253,189 shares of Series AA convertible preferred stock at a price of $ 6.78 per share. As of the issuance date, the estimated fair value of the Series AA convertible preferred stock was $ 4.84 per share, compared with the purchase price per share of $ 6.78 . The difference of $ 58.6 million between the estimated fair value of the stock as of the issuance date and the purchase price was deemed to be additional consideration for the GSK Agreement. As a result, the total upfront payment for accounting purposes allocated to the GSK Agreement was $ 103.6 million. Research and Development Services The GSK Agreement was deemed to be within the scope of ASC 606 because GSK engaged the Company to initially provide research and development services, which are outputs of its ongoing activities, in exchange for consideration. The Company identified the following two distinct performance obligations: (i) research and development services related to the two initial collaboration targets, inclusive of the JSC participation and the manufacture of compounds necessary for providing the research and development services and (ii) a material right for GSK to nominate seven additional collaboration targets for which the Company will perform research and development services until the GSK Option Point. To allocate revenue among the performance obligations, the Company determined standalone selling prices (“SSP”) of each obligation. For the research and development services, the SSP was calculated using a cost-plus margin approach. For the material right, the SSP was calculated by reference to the underlying research and development services expected to be provided and the corresponding expected consideration. All amounts included in the transaction price are allocated to performance obligations proportionate to their SSPs. As of September 30, 2021, the transaction price was deemed to be $ 103.6 million, consisting of the upfront payment of $ 45.0 million under the GSK Agreement and the $ 58.6 million allocated from the GSK Stock Purchase Agreement. Other than the upfront payment and the amounts allocated from the GSK Stock Purchase Agreement, all other contingent consideration that may be earned under the GSK Agreement is subject to uncertainties including but not limited to target addition, research and investigational new drug enabling studies, initiation of clinical trials, and other related achievements. Consequently, the transaction price currently does not include any such contingent consideration that, if included, could result in a probable significant reversal of cumulative revenue when related uncertainties become resolved. The Company will re-evaluate the transaction price at each reporting period. If and when contingent consideration is included in the transaction price, it will be allocated to the two performance obligations proportionate to their SSPs and a cumulative catch up in revenue will be recorded for the portion of the services already completed. The remaining amounts will be deferred and recognized as the services are rendered. The research and development services are transferred as the services are performed, with cost used as the measure of progress compared to total estimated cost to complete. Incurred cost represents work performed, which corresponds with, and thereby best depicts, the transfer of control to the customer. The determination of the percentage of completion requires the Company to estimate the costs to complete the project. The Company makes a detailed estimate of the costs to complete, which is reassessed every reporting period based on the latest project plan and discussions with project teams. If a change in facts or circumstances occurs, the estimate will be adjusted, and the revenue will be recognized based on the revised estimate. The difference between the cumulative revenue recognized based on the previous estimate and the revenue recognized based on the revised estimate would be recognized as an adjustment to revenue in the period in which the change in estimate occurs. The Company recognized revenue related to the research and development services for the two initial targets of $ 2.6 million and $ 1.1 million for the three months ended September 30, 2021 and 2020, respectively, and $ 7.7 million and $ 5.4 million for the nine months ended September 30, 2021 and 2020, respectively. Changes in deferred revenue during the nine months ended September 30, 2021 were as follows (in thousands): Deferred revenue balance at December 31, 2020 $ 95,161 Revenue recognized during the period previously recorded in deferred revenue ( 7,697 ) Deferred revenue balance at September 30, 2021 $ 87,464 Exercise of the License Option In April 2021, GSK exercised the License Option on NY-ESO-1 TCR with Gen-R, a Component Development Program, and will assume sole responsibility for future development and commercialization of the program at its own cost and expense. The Company is entitled to the remaining development and sales milestones up to an aggregate of approximately $ 400.0 million as well as the tiered royalties on future sales of all such products covered by the license granted pursuant to the License Option. The exercise of the License Option was accounted for as a separate license contract for revenue recognition purposes. The Company identified one performance obligation, which was the license delivered to GSK upon the exercise of the License Option and transfer of information and data associated with the license. The Company concluded that the development milestone payments are solely dependent on GSK’s performance and achievement of the specified events and are deemed to be not probable until such development milestones are actually achieved. Therefore, the remaining development milestones are fully constrained and excluded from the transaction price until the respective milestone is achieved. The Company also concluded that sales milestones and royalties relate predominantly to the license granted to GSK. Therefore, they also have been excluded from the transaction price and will be recognized when the related sales occur. At the end of each reporting period, the Company will update its assessment of whether an estimate of variable consideration is constrained and update the estimated transaction price accordingly. As of September 30, 2021, there were no contract assets or contract liabilities related to the license contract. None of the costs to obtain or fulfill the contract were capitalized. No license revenue was recognized for the three and nine months ended September 30, 2021. PACT In June 2020, the Company entered into an agreement (the "PACT Agreement”) with PACT Pharma, Inc. (“PACT”) to jointly develop and test a next generation personalized anti-cancer T cell therapy against solid tumors. The Company paid PACT an upfront non-refundable payment of $ 50.0 million upon execution of the PACT Agreement. In November 2020, the parties agreed to suspend research and development activity under the PACT Agreement, and neither party would be required to conduct any further work under the development plan (including manufacturing development) nor incur any financial obligations (including milestone payments) that might otherwise arise, for as long as the parties continued to negotiate in good faith to resolve the issues that have arisen between them relating to the PACT Agreement. In June 2020 in connection with the entry into the PACT Agreement, the Company also entered into a stock purchase agreement with PACT (“PACT SPA”), pursuant to which the Company purchased 17,806,901 shares of PACT Series C-1 convertible preferred stock at a purchase price of $ 2.81 per share. As of the purchase date, the estimated fair value of the Series C-1 convertible preferred stock was $ 2.05 per share, and the difference between the estimated fair value of the preferred stock as of the purchase date and the purchase price of $ 13.6 million was deemed to be additional consideration for the PACT Agreement and recognized as research and development expense. As a result, the total upfront payment paid in connection with the PACT Agreement was $ 63.6 million and was included in research and development expense. The remaining $ 36.4 million associated with the PACT Series C-1 convertible preferred stock was recorded in other investments. In February 2021, the Company filed a demand for arbitration seeking, among other things, rescission of the PACT Agreement and the PACT SPA and recovery of the consideration paid thereunder. An arbitration hearing has been scheduled to occur in March 2022. NCI License Agreement In December 2020, the Company entered into a license agreement with National Cancer Institute ("NCI") to access certain intellectual property for the development of treatment of human cancers. In connection with this agreement, the Company paid $ 100,000 upfront, and a prorated annual maintenance payment for 2020 of approximately $ 3,100 , for total consideration of approximately $ 103,100 , which was recorded in research and development expense for the year ended December 31, 2020. The Company is also required to pay NCI annual maintenance payments which, may be credited against earned royalties. Under the agreement, the Company may also be required to make certain prespecified development milestone payments up to an aggregate of $ 3.1 million, and prespecified commercial milestone payments up to a maximum aggregate of $ 12.0 million for all licensed products. In June 2021, the Company entered into an amendment to the license agreement with NCI to include additional intellectual property and one additional inventor. In connection with this amendment, the Company paid $ 25,000 upfront, which was recorded in research and development expense. Under the amendment, the Company may also be required to pay prespecified additional development milestone payments that total $ 75,000 .

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Cash Equivalents and Marketable Securities	4. Cash Equivalents and Marketable Securities The fair value and amortized cost of cash equivalents and marketable securities by major security type as of September 30, 2021 and December 31, 2020 are as follows (in thousands): September 30, 2021 Amortized Cost Gross Gross Fair Value Money market funds $ 331,349 $ - $ - $ 331,349 U.S. Treasury securities 249,817 41 ( 97 ) 249,761 U.S. government agency securities 103,950 25 ( 14 ) 103,961 Corporate debt securities 230,724 6 ( 26 ) 230,704 Total cash equivalents and marketable $ 915,840 $ 72 $ ( 137 ) $ 915,775 Classified as: Fair Value Cash equivalents $ 372,847 Short-term marketable securities 225,417 Long-term marketable securities 317,511 Total cash equivalents and marketable $ 915,775 December 31, 2020 Amortized Cost Gross Gross Fair Value Money market funds $ 50,513 $ - $ - $ 50,513 U.S. Treasury securities 202,674 27 - 202,701 U.S. government agency securities 205,558 207 ( 1 ) 205,764 Corporate debt securities 211,086 34 ( 11 ) 211,109 Total cash equivalents and marketable $ 669,831 $ 268 $ ( 12 ) $ 670,087 Classified as: Fair Value Cash equivalents $ 117,879 Short-term marketable securities 472,213 Long-term marketable securities 79,995 Total cash equivalents and marketable $ 670,087 As of September 30, 2021 and December 31, 2020 the fair value of securities held by the Company in an unrealized loss position was $ 312.5 million and $ 132.6 million, respectively, and as of September 30, 2021 and December 31, 2020, securities held by the Company in an unrealized loss position have been in the continuous loss position for less than 12 months. The Company determined that there was no material change in the credit risk of the above investments during the three and nine months ended September 30, 2021. As such, an allowance for credit losses has not been recognized. As of September 30, 2021, the Company does not intend to sell such securities, and it is not more-likely-than-not that the Company will be required to sell the securities prior to the recovery of the amortized cost basis. Gross realized gains and losses were de minimis for the three and nine months ended September 30, 2021 and 2020 and as a result, amounts reclassified out of accumulated other comprehensive (loss) income for the three and nine months ended September 30, 2021 and 2020 were also de minimis . As of September 30, 2021 and December 31, 2020, all of the Company’s marketable securities had a maturity date of two years or less, were available for use and were classified as available-for-sale.

Other Investments

Other Investments	9 Months Ended
	Sep. 30, 2021
Other Investments [Abstract]
Other Investments	5. Other Investments From time to time, the Company makes minority ownership strategic investments. As of both September 30, 2021 and December 31, 2020, the aggregate carrying amounts of the Company’s strategic investments in non-publicly traded companies were $ 83.4 million. These investments are measured at initial cost, minus impairment, if any, and plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. There were no adjustments recorded to the carrying amount for other investments for the three and nine months ended September 30, 2021 and 2020. In November 2020, the Company made a strategic equity investment of $ 13.0 million in Outpace Bio, Inc. (“Outpace”), a privately-held company, which represented a minority ownership interest at the time of the strategic investment. Outpace is engaged in the research and development of protein and cell technology platforms and has financed its activities via issuances of preferred stock. The Company determined that Outpace is a variable interest entity (“VIE”) as the at-risk equity holders, as a group, lack the characteristics of a controlling financial interest. The Company does not have majority voting rights, representation on Outpace’s board of directors or the power to direct the activities of this entity and, therefore, it is not the primary beneficiary. As of September 30, 2021 and December 31, 2020, the carrying value of the Company’s investment in Outpace i s $ 13.0 million, which is recorded in other investments.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	6. Fair Value Measurements The following table sets forth the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2021 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 331,349 $ - $ - $ 331,349 U.S. Treasury securities - 249,761 - 249,761 U.S. government agency securities - 103,961 - 103,961 Corporate debt securities - 230,704 - 230,704 Equity warrant investment - - 1,124 1,124 Total financial assets $ 331,349 $ 584,426 $ 1,124 $ 916,899 Financial liabilities: Success payment liabilities $ - $ - $ 25,116 $ 25,116 Total financial liabilities $ - $ - $ 25,116 $ 25,116 December 31, 2020 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 50,513 $ - $ - $ 50,513 U.S. Treasury securities - 202,701 - 202,701 U.S. government agency securities - 205,764 - 205,764 Corporate debt securities - 211,109 - 211,109 Equity warrant investment - - 1,323 1,323 Total financial assets $ 50,513 $ 619,574 $ 1,323 $ 671,410 Financial liabilities: Success payment liabilities $ - $ - $ 5,773 $ 5,773 Total financial liabilities $ - $ - $ 5,773 $ 5,773 The Company measures the fair value of money market funds based on quoted prices in active markets for identical assets or liabilities. The Level 2 marketable securities include U.S. Treasury and government agency securities and corporate debt securities. The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models. Inputs utilized include market pricing based on real-time trade data for the same or similar securities and other significant inputs derived from or corroborated by observable market data. The Level 3 financial instruments include an equity warrant investment and success payment liabilities. The Company’s Level 3 financial instruments are valued using valuation models which include the Black-Scholes model for valuing the equity warrant investment and a Monte Carlo simulation for the success payment liabilities. To determine the estimated fair value of the success payments, the Company uses a Monte Carlo simulation methodology which models the future movement of stock prices based on several key variables combined with empirical knowledge of the process governing the behavior of the stock price. The following variables were incorporated in the estimated fair value of the success payment liabilities: fair value of the Company's common stock (Series A convertible preferred stock, prior to IPO), expected volatility, risk-free interest rate and the estimated number and timing of valuation measurement dates on the basis of which payments may be triggered. The computation of expected volatility was estimated based on available information about the historical volatility of stocks of similar publicly traded companies for a period matching the expected term assumption. The following assumptions were incorporated into the calculation of the estimated fair value of the Fred Hutch success payment liability: September 30, December 31, 2021 2020 Fair value of common stock (Series A convertible preferred stock) $ 14.80 $ 9.07 Risk-free interest rate 0.07 % - 2.24 % 0.10 % - 1.52 % Expected volatility 75 % 80 % Expected term (in years) 0.71 - 6.22 1.00 - 6.97 The following assumptions were incorporated into the calculation of the estimated fair value of the Stanford success payment liability: September 30, December 31, 2021 2020 Fair value of common stock (Series A convertible preferred stock) $ 14.80 $ 9.07 Risk-free interest rate 0.07 % - 2.24 % 0.10 % - 1.53 % Expected volatility 75 % 80 % Expected term (in years) 0.71 - 8.00 1.00 - 8.75 The Company utilizes estimates and assumptions in determining the estimated success payment liabilities and associated expense. A small change in the valuation of the Company’s common stock may have a relatively large change in the estimated fair value of the success payment liability and associated expense. The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial assets and liabilities (in thousands): Equity Warrant Success Payment Balance at December 31, 2020 $ 1,323 $ 5,773 Change in fair value (1) ( 199 ) 19,343 Balance at September 30, 2021 $ 1,124 $ 25,116 (1) The change in fair value associated with the equity warrant investment is recorded in other income (expense), net and the change in fair value associated with success payment liabilities is recorded in research and development expense.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	7. Leases The Company’s lease portfolio is comprised of operating leases for laboratory, office and manufacturing facilities located in South San Francisco, California, Seattle, Washington and Bothell, Washington with contractual periods expiring between December 2021 and March 2031. In addition to minimum rent, the leases require payment of real estate taxes, insurance, common area maintenance charges and other executory costs. These additional charges are considered variable lease costs and are recognized in the period in which the costs are incurred. The following table summarizes the Company’s future minimum operating lease commitments, including expected lease incentives to be received, as of September 30, 2021 (in thousands): Year ending December 31: 2021 (remaining three months) $ 2,419 2022 10,698 2023 11,018 2024 11,347 2025 11,859 Thereafter 60,302 Total undiscounted lease payments 107,643 Less: imputed interest ( 33,422 ) Less: tenant improvement allowances ( 6,993 ) Total operating lease liabilities $ 67,228 Reported as of September 30, 2021: Short-term portion of lease liabilities (included in accrued liabilities and other current liabilities) $ 1,544 Operating lease liabilities, non-current 65,684 Total $ 67,228 The operating lease costs for all operating leases were $ 2.4 million and $ 2.9 million for the three months ended September 30, 2021 and 2020, respectively, and $ 7.1 m illion and $ 8.3 million for the nine months ended September 30, 2021 and 2020, respectively. The operating lease costs and total commitments for short-term leases were de minimis for the three and nine months ended September 30, 2021 and 2020. Variable lease costs for operating leases were $ 1.4 million and $ 0.5 million for the three months ended September 30, 2021 and 2020, respectively, and $ 3.7 million and $ 1.5 million for the nine months ended September 30, 2021 and 2020, respectively. In May 2021, the Company entered into a sublease, whereby the Company agreed to sublease approximately 11,000 rentable square feet of its space in South San Francisco, California currently leased by the Company. The sublease commenced in August 2021, is classified as an operating lease and will expire in March 2031. The monthly fixed payment due to the Company is $ 0.1 million, subject to annual rent increases in accordance with the contract. In September 2021, the Company entered into a sublease with Sonoma Biotherapeutics, Inc. ("Sonoma"), a related party, whereby the Company agreed to sublease approximately 18,000 rentable square feet of space in South San Francisco, California currently leased by the Company. See Note 13, Related-Party Transactions. As a part of the sublease, in September 2021, the Company received a $ 4.6 million tenant improvement contribution payment, which will be recognized over the term of the sublease. The sublease commenced in September 2021, is classified as an operating lease and will expire in March 2031. The monthly fixed payment due to the Company is $ 0.1 million, subject to annual rent increases in accordance with the contract.

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2021
Stockholders' Equity Note [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock In March 2020, the Company sold 42,905,042 shares of its Series C convertible preferred stock at a price of $ 11.49 per share for proceeds of $ 492.5 million, net of issuance costs of $ 0.5 million. In March 2020, the Company repurchased 546,806 shares of its Series A convertible preferred stock from a related party for a purchase price of $ 4.2 million. Upon the closing of the IPO, 194,474,431 shares of convertible preferred stock then outstanding converted into an equivalent number of shares of common stock. As of September 30, 2021, no shares of convertible preferred stock were outstanding.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' Equity	9. Stockholders' Equity The Company amended and restated its certificate of incorporation effective June 2021, increasing the number of shares the Company has the authority to issue to 510.0 million shares, of which 500.0 million are common shares and 10.0 million shares are preferred stock. Preferred Stock The Company is authorized to issue 10,000 ,000 shares of preferred stock, par value $ 0.0001 per share. As of September 30, 2021, no shares of preferred stock were outstanding. Common Stock As of September 30, 2021 and December 31, 2020, there were 239,789,419 shares and 15,569,788 shares of the Company’s common stock outstanding, respectively, excluding 3,575,002 shares and 7,562,503 shares, respectively, of restricted stock awards (“RSAs”) outstanding that are subject to vesting requirements. In March 2020, the Company repurchased 2,032,166 shares of its common stock from a related party for a purchase price of $ 11.8 million.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-based Compensation	10. Stock-based Compensation 2021 Equity Incentive Plan In June 2021, the Company adopted the 2021 Equity Incentive Plan ("2021 Plan"), which on the date of the underwriting agreement related to the Company's IPO became effective with an initial reserve of 26,662,087 shares, plus any shares subject to outstanding awards granted under the 2018 Equity Incentive Plan ("2018 Plan") that, on or after the effectiveness of the 2021 Plan, terminate or expire before exercise or settlement, are not issued because the award is settled in cash, are forfeited because of the failure to vest, or are reacquired or withheld (or not issued) to satisfy a tax withholding obligation or the purchase or exercise price. In addition, the number of shares reserved for issuance under the 2021 Plan will automatically increase on January 1 of each year for a period of ten years, beginning on January 1, 2022 and continuing through January 1, 2031, in an amount equal to (1) 5 % of the total number of shares of the Company's common stock outstanding on December 31 of the immediately preceding year, or (2) a lesser number of shares determined by the Company's board of directors no later than December 31 of the immediately preceding year. Under the 2021 Plan, the Company may grant incentive stock options, non-statutory stock options, RSAs, restricted stock units, stock appreciation rights, performance awards and other stock-based awards. Terms of stock awards, including vesting requirements, are determined by the Company's board of directors or by a committee authorized by the Company's board of directors, subject to provisions of the 2021 Plan. The term of any stock option granted under the 2021 Plan cannot exceed ten years . Generally, awards granted by the Company vest over four years , but may be granted with different vesting terms. In conjunction with adopting the 2021 Plan, the Company discontinued the 2018 Plan with respect to new equity awards. As of September 30, 2021, 24.9 million shares were available for future issuance pursuant to the 2021 Plan. 2021 Employee Stock Purchase Plan In June 2021, the Company adopted the 2021 Employee Stock Purchase Plan ("2021 ESPP"), which became effective immediately prior to the execution of the underwriting agreement related to the Company's IPO with an initial reserve of 2,470,000 shares. The 2021 ESPP allows eligible employees to purchase shares of the Company's common stock at a discount through payroll deductions of up to 15 % of their earnings, subject to plan limitations. Unless otherwise determined by the Company's board of directors, employees are able to purchase shares at 85 % of the lower of the fair market value of the Company's common stock on the first date of an offering or on the purchase date. The number of shares of the Company's common stock reserved for issuance under the 2021 ESPP will automatically increase on January 1 of each year for a period of ten years, beginning on January 1, 2022 and continuing through January 1, 2031, by the lesser of (1) 1 % of the total number of shares of the Company's common stock outstanding on December 31 of the immediately preceding year, and (2) 4,940,000 shares; provided, however, that the Company's board of directors may act to provide a lesser increase in number of shares. The Company may specify offerings with durations not more than 27 months and may specify shorter purchase periods within each offering. No shares have been issued under the 2021 ESPP as of September 30, 2021. 2018 Equity Incentive Plan In 2018, the Company established the 2018 Plan under which it may grant incentive stock options, non-s tatutory stock options, RSAs, restricted stock units, stock appreciation rights and other stock-based awards. Terms of stock awards, including vesting requirements, were determined by the Company's board of directors or by a committee authorized by the Company's board of directors, subject to provisions of the 2018 Plan. The term of any stock option granted under the 2018 Plan cannot exceed ten years. Generally, awards granted by the Company vest over four years, but may have been granted with different vesting terms. Pursuant to the terms of the 2021 Plan, any shares subject to outstanding options originally granted under the 2018 Plan that terminate, expire or lapse for any reason without the delivery of shares to the holder thereof shall become available for issuance pursuant to awards granted under the 2021 Plan. While no shares are available for future issuance under the 2018 Plan, it continues to govern outstanding equity awards granted thereunder. Stock-based Compensation Expense Stock-based compensation expense by classification included with in the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Research and development $ 2,673 $ 1,965 $ 12,615 $ 9,033 General and administrative 10,369 7,783 28,408 11,257 Total stock-based compensation expense $ 13,042 $ 9,748 $ 41,023 $ 20,290 Stock-based compensation expense for the nine months ended September 30, 2021 includes the impact of an award accelerated in connection with the Company's IPO resulting in stock-based compensation expense of $ 2.6 millio n. At September 30, 2021, total stock-based compensation cost related to unvested awards not yet recognized was $ 122.5 million, which is expected to be recognized over a remaining weighted-average period of 2.46 years. Restricted Stock Awards A summary of the Company’s RSAs activity was as follows: Number of Weighted-Average Unvested shares as of December 31, 2020 7,562,503 $ 0.0001 Vested ( 3,968,751 ) $ 0.0001 Forfeited ( 18,750 ) $ 0.0001 Unvested shares as of September 30, 2021 3,575,002 $ 0.0001 Stock Options A summary of the Company’s stock option activity was as follows: Number of Weighted- Weighted- Aggregate Options outstanding as of December 31, 2020 34,413,889 $ 3.33 Granted 12,521,501 $ 9.00 Exercised ( 776,449 ) $ 2.63 Canceled or forfeited ( 2,597,874 ) $ 4.00 Options outstanding as of September 30, 2021 43,561,067 $ 4.93 7.97 $ 432,916 Options exercisable as of September 30, 2021 24,397,665 $ 3.29 7.17 $ 280,840 The fair value of stock options granted to employees, directors and consultants was estimated on the date of grant using the Black-Scholes option pricing model using the following weighted-average assumptions: Nine Months Ended September 30, 2021 2020 Risk-free interest rate 0.79 % 0.99 % Expected volatility 79 % 75 % Expected term (in years) 6.10 6.25 Expected dividend yield 0 % 0 % The weighted-average grant date fair value of options granted for the nine months ended September 30, 2021 and 2020 was $ 6.55 per share and $ 3.21 per share, respectively.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Loss Per Share	11. Net Loss Per Share Basic and diluted net loss per share attributed to common stockholders is calculated by dividing net loss attributed to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. The Company’s potentially dilutive shares, which include preferred stock, unvested RSAs and options to purchase common stock, are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. Potentially dilutive common shares have been excluded from the diluted net loss per common share computations in all periods presented because such securities have an anti-dilutive effect on net loss per common share due to the Company’s net losses. There are no reconciling items used to calculate the weighted-average number of total common shares outstanding for basic and diluted net loss per common share. As of September 30, 2021 and 2020 potentially dilutive securities were as follows: September 30, 2021 2020 Convertible preferred stock - 194,474,431 Unvested RSAs 3,575,002 9,486,462 Options to purchase common stock 43,561,067 32,358,470 Total 47,136,069 236,319,363

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Collaboration and License Agreements The Company has entered into certain collaboration and license agreements, including those identified in Note 3, Collaboration, License and Success Payment Agreements above, with third parties that include the funding of certain development, manufacturing and commercialization efforts with the potential for future milestone and royalty payments upon the achievement of pre-established developmental, regulatory and/or commercial milestones. The Company’s obligation to fund these efforts is contingent upon continued involvement in the programs and/or the lack of any adverse events which could cause the discontinuance of the programs. Due to the nature of these agreements, the future potential payments are inherently uncertain, and accordingly no amounts have been recorded for the potential future achievement of these targets as of September 30, 2021 and December 31, 2020 .

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	13. Related-Party Transactions In September 2021, the Company entered into a sublease with Sonoma, with whom the Company has common stockholders with board seats. As a part of the sublease, a $ 4.6 million tenant improvement contribution payment was made by Sonoma, which will be recognized over the term of the sublease. As of September 30, 2021, accrued liabilities and other current liabilities of $ 0.5 million and other non-current liabilities of $ 4.1 million were in connection with the sublease with Sonoma. Income of $ 0.5 million and $ 1.2 million was recognized in other operating income, net for the three and nine months ended September 30, 2021, respectively. See Note 7, Leases , for more detail on the Sonoma sublease. The Company is party to the GSK Agreement with GSK, who is a holder of more than 10 % of the Company’s equity. See Note 3, Collaboration, License and Success Payment Agreements . Deferred revenue of $ 5.6 million and $ 6.1 million as of September 30, 2021 and December 31, 2020, respectively, and deferred revenue, net of current portion of $ 81.9 million and $ 89.1 million as of September 30, 2021 and December 31, 2020, respectively, were in connection with the GSK Agreement. Revenue recognized in connection with the GSK agreement was $ 2.6 million and $ 1.1 million for the three months ended September 30, 2021 and 2020, respectively, and $ 7.7 million and $ 5.4 million for the nine months ended September 30, 2021 and 2020, respectively. In March 2020, the Company repurchased 546,806 shares of its Series A convertible preferred stock and 2,032,166 shares of its common stock from a related party. See Note 8, Convertible Preferred Stock and Note 9, Stockholders' Equity .

Asset Acquisition

Asset Acquisition	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Asset Acquisition	14. Asset Acquisition In May 2020, the Company completed the acquisition of 100 % of the outstanding equity of Immulus, Inc. (“Immulus”), a company focused on developing technology platforms that enable the development and production of cell therapeutics. As consideration for the acquisition, the Company paid $ 3.5 million in cash and issued an aggregate of 688,463 shares of its common stock, with an estimated fair value of $ 4.0 million. The Company also incurred $ 0.5 million of direct expenses, for total consideration of $ 8.0 million. The Company concluded the acquisition did not meet the accounting definition of a business as inputs were acquired, but no processes or outputs were acquired. Consequently, the Company accounted for the transaction as an asset acquisition with the value concentrated in IPR&D. The following table summarizes the fair value of assets acquired (in thousands): Other assets $ 487 In-process research and development (IPR&D) 7,528 Total assets acquired $ 8,015 The amount allocated to the IPR&D asset was charged to research and development expenses for the nine months ended September 30, 2020 as this asset had no alternative future use at the time of the acquisition transaction. In addition, the Company is also required to make milestone payments of up to $ 37.0 million to the former stockholders of Immulus upon successful completion of specified development milestones. Triggering of these milestones payments was not considered probable as of the date of the acquisition, and no expense has been recorded for these milestones for the three and nine months ended September 30, 2021 and 2020.

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The condensed consolidated financial statements include the accounts of Lyell Immunopharma, Inc. and its wholly-owned subsidiary. All significant intercompany transactions and balances are eliminated in consolidation. The condensed consolidated balance sheet as of December 31, 2020 included herein was derived from the audited consolidated financial statements as of that date. Certain information and footnote disclosures typically included in the Company's audited consolidated financial statements have been condensed or omitted. The accompanying unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s financial position, results of operations and cash flows for the periods presented, but are not necessarily indicative of results to be expected for any future annual or interim period. These condensed consolidated financial statements should be read in conjunction with the Company's audited financial statements and notes included in the prospectus the Company filed with the Securities and Exchange Commission on June 21, 2021 pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the "Prospectus").
Use of Estimates	Use of Estimates The preparation of the Company’s condensed consolidated financial statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect reported amounts and related disclosures. Specific accounts that require management estimates include, but are not limited to, stock-based compensation, valuation of success payments, revenue recognition and accrued expenses. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Actual results could differ materially from those estimates.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off-Balance Sheet Risk The Company maintains its cash and cash equivalents and restricted cash with high quality, accredited financial institutions. These amounts, at times, may exceed federally insured limits. The Company also makes short-term investments in money market funds, U.S. Treasury securities, U.S. government agency securities and corporate debt securities, which can be subject to certain credit risk. However, the Company mitigates the risks by investing in high-grade instruments, limiting exposure to any one issuer or type of investment and monitoring the ongoing creditworthiness of the financial institutions and issuers. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to significant risk on these funds. The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.
Significant Accounting Policies	Significant Accounting Policies There have been no significant changes to the accounting policies during the nine months ended September 30, 2021, as compared to the significant accounting policies described in Note 2 of the “Notes to Financial Statements” in the Company’s audited financial statements included in the Prospectus, with the exception of revenue recognition related to licenses of intellectual property during the nine months ended September 30, 2021.
Revenue	Revenue The Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements within the scope of Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers , ("ASC 606") the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the performance obligation is satisfied. In applying the ASC 606 framework, the Company must apply judgment to determine the nature of the promises within a revenue contract and whether those promises represent distinct performance obligations. In determining the transaction price, the Company does not include amounts subject to uncertainties unless it is probable that there will be no significant reversal of cumulative revenue when the uncertainty is resolved. Milestone and other forms of variable consideration that the Company may earn are subject to significant uncertainties of research and development related achievements, which generally are deemed not probable until such milestones are actually achieved. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). Additionally, the Company develops assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. The Company then allocates the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation, for which it recognizes revenue as or when the performance obligations are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the variable consideration and any related constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis. Under the Company’s license agreements, the Company grants the license to a customer as it exists at the point of transfer and the nature of the license is a right to use the Company’s intellectual property as transferred. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time.

Collaboration, License and Su_2

Collaboration, License and Success Payment Agreements (Tables)	9 Months Ended
	Sep. 30, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Summary of Changes in Deferred Revenue	Changes in deferred revenue during the nine months ended September 30, 2021 were as follows (in thousands): Deferred revenue balance at December 31, 2020 $ 95,161 Revenue recognized during the period previously recorded in deferred revenue ( 7,697 ) Deferred revenue balance at September 30, 2021 $ 87,464
Fred Hutch [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Schedule of Aggregate Potential Success Payments	The following table summarizes the aggregate potential success payments, which are payable to Fred Hutch in cash or cash equivalents or, at the Company’s discretion, publicly-tradeable shares of the Company’s common stock: Multiple of initial equity value at issuance 10 x 20 x 30 x 40 x 50 x Per share common stock price required for payment $ 18.29 $ 36.58 $ 54.86 $ 73.15 $ 91.44 Aggregate success payment(s) (in millions) $ 10 $ 40 $ 90 $ 140 $ 200
Stanford [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Schedule of Aggregate Potential Success Payments	The following table summarizes the aggregate potential success payments, which are payable to Stanford in cash or cash equivalents or, at the Company’s discretion, publicly-tradeable shares of the Company’s common stock: Multiple of initial equity value at issuance 10 x 20 x 30 x 40 x 50 x Per share common stock price required for payment $ 18.29 $ 36.58 $ 54.86 $ 73.15 $ 91.44 Aggregate success payment(s) (in millions) $ 10 $ 40 $ 90 $ 140 $ 200

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Schedule of Fair Value and Amortized Cost of Cash Equivalents and Marketable Securities	The fair value and amortized cost of cash equivalents and marketable securities by major security type as of September 30, 2021 and December 31, 2020 are as follows (in thousands): September 30, 2021 Amortized Cost Gross Gross Fair Value Money market funds $ 331,349 $ - $ - $ 331,349 U.S. Treasury securities 249,817 41 ( 97 ) 249,761 U.S. government agency securities 103,950 25 ( 14 ) 103,961 Corporate debt securities 230,724 6 ( 26 ) 230,704 Total cash equivalents and marketable $ 915,840 $ 72 $ ( 137 ) $ 915,775 Classified as: Fair Value Cash equivalents $ 372,847 Short-term marketable securities 225,417 Long-term marketable securities 317,511 Total cash equivalents and marketable $ 915,775 December 31, 2020 Amortized Cost Gross Gross Fair Value Money market funds $ 50,513 $ - $ - $ 50,513 U.S. Treasury securities 202,674 27 - 202,701 U.S. government agency securities 205,558 207 ( 1 ) 205,764 Corporate debt securities 211,086 34 ( 11 ) 211,109 Total cash equivalents and marketable $ 669,831 $ 268 $ ( 12 ) $ 670,087 Classified as: Fair Value Cash equivalents $ 117,879 Short-term marketable securities 472,213 Long-term marketable securities 79,995 Total cash equivalents and marketable $ 670,087

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	9 Months Ended
	Sep. 30, 2021
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2021 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 331,349 $ - $ - $ 331,349 U.S. Treasury securities - 249,761 - 249,761 U.S. government agency securities - 103,961 - 103,961 Corporate debt securities - 230,704 - 230,704 Equity warrant investment - - 1,124 1,124 Total financial assets $ 331,349 $ 584,426 $ 1,124 $ 916,899 Financial liabilities: Success payment liabilities $ - $ - $ 25,116 $ 25,116 Total financial liabilities $ - $ - $ 25,116 $ 25,116 December 31, 2020 Level 1 Level 2 Level 3 Total Financial assets: Money market funds $ 50,513 $ - $ - $ 50,513 U.S. Treasury securities - 202,701 - 202,701 U.S. government agency securities - 205,764 - 205,764 Corporate debt securities - 211,109 - 211,109 Equity warrant investment - - 1,323 1,323 Total financial assets $ 50,513 $ 619,574 $ 1,323 $ 671,410 Financial liabilities: Success payment liabilities $ - $ - $ 5,773 $ 5,773 Total financial liabilities $ - $ - $ 5,773 $ 5,773
Changes in the Estimated Fair Value of Level 3 Financial Assets and Liabilities	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial assets and liabilities (in thousands): Equity Warrant Success Payment Balance at December 31, 2020 $ 1,323 $ 5,773 Change in fair value (1) ( 199 ) 19,343 Balance at September 30, 2021 $ 1,124 $ 25,116 (1) The change in fair value associated with the equity warrant investment is recorded in other income (expense), net and the change in fair value associated with success payment liabilities is recorded in research and development expense.
Fred Hutch
Summary of Estimated Fair Value of Success Payment Liability Assumptions	The following assumptions were incorporated into the calculation of the estimated fair value of the Fred Hutch success payment liability: September 30, December 31, 2021 2020 Fair value of common stock (Series A convertible preferred stock) $ 14.80 $ 9.07 Risk-free interest rate 0.07 % - 2.24 % 0.10 % - 1.52 % Expected volatility 75 % 80 % Expected term (in years) 0.71 - 6.22 1.00 - 6.97
Stanford
Summary of Estimated Fair Value of Success Payment Liability Assumptions	The following assumptions were incorporated into the calculation of the estimated fair value of the Stanford success payment liability: September 30, December 31, 2021 2020 Fair value of common stock (Series A convertible preferred stock) $ 14.80 $ 9.07 Risk-free interest rate 0.07 % - 2.24 % 0.10 % - 1.53 % Expected volatility 75 % 80 % Expected term (in years) 0.71 - 8.00 1.00 - 8.75

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Summary of Operating Lease Commitments Including Expected Lease Incentives to be Received	The following table summarizes the Company’s future minimum operating lease commitments, including expected lease incentives to be received, as of September 30, 2021 (in thousands): Year ending December 31: 2021 (remaining three months) $ 2,419 2022 10,698 2023 11,018 2024 11,347 2025 11,859 Thereafter 60,302 Total undiscounted lease payments 107,643 Less: imputed interest ( 33,422 ) Less: tenant improvement allowances ( 6,993 ) Total operating lease liabilities $ 67,228 Reported as of September 30, 2021: Short-term portion of lease liabilities (included in accrued liabilities and other current liabilities) $ 1,544 Operating lease liabilities, non-current 65,684 Total $ 67,228

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Schedule of Stock-based Compensation Expense by Classification	Stock-based compensation expense by classification included with in the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Research and development $ 2,673 $ 1,965 $ 12,615 $ 9,033 General and administrative 10,369 7,783 28,408 11,257 Total stock-based compensation expense $ 13,042 $ 9,748 $ 41,023 $ 20,290
Summary of RSA Activity	A summary of the Company’s RSAs activity was as follows: Number of Weighted-Average Unvested shares as of December 31, 2020 7,562,503 $ 0.0001 Vested ( 3,968,751 ) $ 0.0001 Forfeited ( 18,750 ) $ 0.0001 Unvested shares as of September 30, 2021 3,575,002 $ 0.0001
Summary of Stock Option Activity	A summary of the Company’s stock option activity was as follows: Number of Weighted- Weighted- Aggregate Options outstanding as of December 31, 2020 34,413,889 $ 3.33 Granted 12,521,501 $ 9.00 Exercised ( 776,449 ) $ 2.63 Canceled or forfeited ( 2,597,874 ) $ 4.00 Options outstanding as of September 30, 2021 43,561,067 $ 4.93 7.97 $ 432,916 Options exercisable as of September 30, 2021 24,397,665 $ 3.29 7.17 $ 280,840
Schedule of Assumptions Used in Black-Scholes Option-Pricing Model for Estimating Fair Value of Stock Options Granted	The fair value of stock options granted to employees, directors and consultants was estimated on the date of grant using the Black-Scholes option pricing model using the following weighted-average assumptions: Nine Months Ended September 30, 2021 2020 Risk-free interest rate 0.79 % 0.99 % Expected volatility 79 % 75 % Expected term (in years) 6.10 6.25 Expected dividend yield 0 % 0 %

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Schedule of Potentially Dilutive Securities	As of September 30, 2021 and 2020 potentially dilutive securities were as follows: September 30, 2021 2020 Convertible preferred stock - 194,474,431 Unvested RSAs 3,575,002 9,486,462 Options to purchase common stock 43,561,067 32,358,470 Total 47,136,069 236,319,363

Asset Acquisition (Tables)

Asset Acquisition (Tables)	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Summary of Fair Value of Assets Acquired	The following table summarizes the fair value of assets acquired (in thousands): Other assets $ 487 In-process research and development (IPR&D) 7,528 Total assets acquired $ 8,015

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Description Of Organization [Line Items]
Proceeds from initial public offering, net of underwriters' discounts and commissions		$ 391,802
Outstanding convertible preferred stock (in shares)		0		194,474,000
Convertible Preferred Stock
Description Of Organization [Line Items]
Conversion of preferred stock into common stock (in shares)		194,474,000
Outstanding convertible preferred stock (in shares)			194,474,000	194,474,000	194,474,000	152,116,000
Common Stock
Description Of Organization [Line Items]
Conversion of preferred stock into common stock (in shares)		194,474,000
IPO | Convertible Preferred Stock
Description Of Organization [Line Items]
Conversion of preferred stock into common stock (in shares)	194,474,431
IPO | Common Stock
Description Of Organization [Line Items]
Issuance of common stock, shares	25,000,000
Payments of underwriting discounts and commissions	$ 29,800
Proceeds from initial public offering, net of underwriters' discounts and commissions	$ 391,800
Common stock price per share	$ 17
Offering expenses	$ 3,400
IPO | Common Stock Including Additional Paid in Capital
Description Of Organization [Line Items]
Converted preferred stock	$ 1,000,000

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2021
Concentration risk, credit risk, financial instruments, off-balance sheet risk	no

Collaboration, License and Su_3

Collaboration, License and Success Payment Agreements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2021	Apr. 30, 2021	Jul. 31, 2019	May 31, 2019
Fred Hutch Collaboration Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement expense	$ 1,100,000	$ 1,000,000		$ 3,100,000	$ 3,000,000
Fair value of success payments net	21,700,000			21,700,000		$ 8,000,000
Success payment expense	$ (800,000)	1,200,000		$ 14,100,000	3,800,000
Estimated fair value per share	$ 1.83			$ 1.83
Fred Hutch Collaboration Agreement [Member] | Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 10,000,000
Share issued price per share	18.29			$ 18.29
Fred Hutch Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 200,000,000
Share issued price per share	$ 91.44			$ 91.44
Fred Hutch Collaboration Agreement [Member] | Maximum [Member] | 50x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 200,000,000
Stanford Collaboration Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement expense	$ 800,000	0		2,300,000	0
Fair value of success payments net	23,100,000			23,100,000		8,900,000
Success payment expense	$ 1,000,000	0		$ 5,200,000	0
Estimated fair value per share	$ 1.83			$ 1.83
Stanford Collaboration Agreement [Member] | Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 10,000,000
Share issued price per share	18.29			$ 18.29
Stanford Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 200,000,000
Share issued price per share	$ 91.44			$ 91.44
Stanford Collaboration Agreement [Member] | Maximum [Member] | 50x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate success payment				$ 200,000,000
GSK Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Agreement upfront consideration	$ 45,000,000			45,000,000					$ 45,000,000
Maximum potential future development and sales milestone payment								$ 400,000,000
Maximum potential future technology validation milestone payment	200,000,000			200,000,000
Transaction price, revenue contract				103,600,000
Revenue recognized	2,600,000	$ 1,100,000		7,700,000	$ 5,400,000
Additional allocated upfront consideration				58,600,000
GSK License Contract [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract assets	0			0
Contract liabilities	0			0
Revenue recognized	0			0
PACT Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total upfront payment for collaboration agreement			$ 63,600,000
License fees			$ 50,000,000
Shares purchased			17,806,901
Share price			$ 2.81
Estimated shares purchased, fair value per share			$ 2.05
Additional allocated upfront purchase price			$ 13,600,000
PACT Agreement [Member] | Other Investment [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Strategic equity investment			$ 36,400,000
G S K Already in Agreement [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum potential future development and sales milestone payment	400,000,000			400,000,000
G S K Not Already in Agreement [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum potential future development and sales milestone payment	$ 900,000,000			$ 900,000,000
NCI License Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Agreement upfront consideration						100,000	$ 25,000
Maintenance cost						3,100
Total consideration						103,100
Prespecified development milestone payment						3,100,000	$ 75,000
Pre-specified commercial milestone payment						$ 12,000,000
Series AA Convertible Preferred Stock | GSK Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Share price										$ 6.78
Total upfront consideration										$ 103,600,000
Shares, issued										30,253,189
Estimated fair value per share	$ 4.84			$ 4.84
Share price	$ 6.78			$ 6.78
Additional allocated upfront consideration				$ 58,600,000

Collaboration, License and Su_4

Collaboration, License and Success Payment Agreements - Schedule of Aggregate Potential Success Payments (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)$ / shares
Fred Hutch [Member] | 10x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	10
Share issued price per share | $ / shares	$ 18.29
Aggregate success payment | $	$ 10
Fred Hutch [Member] | 20x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	20
Share issued price per share | $ / shares	$ 36.58
Aggregate success payment | $	$ 40
Fred Hutch [Member] | 30x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	30
Share issued price per share | $ / shares	$ 54.86
Aggregate success payment | $	$ 90
Fred Hutch [Member] | 40x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	40
Share issued price per share | $ / shares	$ 73.15
Aggregate success payment | $	$ 140
Fred Hutch [Member] | 50x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	50
Share issued price per share | $ / shares	$ 91.44
Aggregate success payment | $	$ 200
Stanford [Member] | 10x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	10
Share issued price per share | $ / shares	$ 18.29
Aggregate success payment | $	$ 10
Stanford [Member] | 20x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	20
Share issued price per share | $ / shares	$ 36.58
Aggregate success payment | $	$ 40
Stanford [Member] | 30x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	30
Share issued price per share | $ / shares	$ 54.86
Aggregate success payment | $	$ 90
Stanford [Member] | 40x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	40
Share issued price per share | $ / shares	$ 73.15
Aggregate success payment | $	$ 140
Stanford [Member] | 50x [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Multiple of initial equity value at issuance	50
Share issued price per share | $ / shares	$ 91.44
Aggregate success payment | $	$ 200

Collaboration, License and Su_5

Collaboration, License and Success Payment Agreements - Summary of Changes in Deferred Revenue (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
GSK License Contract [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue balance at September 30, 2021	$ 0
GSK Research and Development Services Contract [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue balance at December 31, 2020	95,161
Revenue recognized during the period previously recorded in deferred revenue	(7,697)
Deferred revenue balance at September 30, 2021	$ 87,464

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Schedule of Fair Value and Amortized Cost of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 915,840	$ 669,831
Gross Unrealized Gains	72	268
Gross Unrealized Losses	(137)	(12)
Fair Value	915,775	670,087
Short-term Marketable Securities [Member]
Marketable Securities [Line Items]
Fair Value	225,417	472,213
Long-term Marketable Securities [Member]
Marketable Securities [Line Items]
Fair Value	317,511	79,995
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	249,817	202,674
Gross Unrealized Gains	41	27
Gross Unrealized Losses	(97)
Fair Value	249,761	202,701
U.S. Government Agency Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	103,950	205,558
Gross Unrealized Gains	25	207
Gross Unrealized Losses	(14)	(1)
Fair Value	103,961	205,764
Corporate Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	230,724	211,086
Gross Unrealized Gains	6	34
Gross Unrealized Losses	(26)	(11)
Fair Value	230,704	211,109
Money Market Funds [Member]
Marketable Securities [Line Items]
Amortized Cost	331,349	50,513
Fair Value	331,349	50,513
Cash Equivalents [Member]
Marketable Securities [Line Items]
Fair Value	$ 372,847	$ 117,879

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Fair value of securities in unrealized loss position	$ 312.5	$ 132.6
Securities, continuous unrealized loss position, less than twelve months, fair value	$ 312.5	$ 132.6
Maximum [Member]
Marketable Securities [Line Items]
Marketable securities maturity period	2 years	2 years

Other Investments - Additional

Other Investments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Nov. 30, 2020
Schedule of Investments [Line Items]
Other Investments	$ 83,448		$ 83,448		$ 83,448
Adjustments To The Carrying Amount For Other Investments	0	$ 0	0	$ 0
Outpace Bio Inc [Member]
Schedule of Investments [Line Items]
Strategic equity investment	$ 13,000		$ 13,000		$ 13,000	$ 13,000

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financial liabilities:
Success payment liabilities	$ 25,116	$ 5,773
Fair Value, Measurements, Recurring
Financial assets:
Financial assets, fair value	916,899	671,410
Financial liabilities:
Success payment liabilities	25,116	5,773
Total financial liabilities	25,116	5,773
Fair Value, Measurements, Recurring | Money Market Funds [Member]
Financial assets:
Financial assets, fair value	331,349	50,513
Fair Value, Measurements, Recurring | U.S. Treasury Securities [Member]
Financial assets:
Financial assets, fair value	249,761	202,701
Fair Value, Measurements, Recurring | U.S. government agency securities
Financial assets:
Financial assets, fair value	103,961	205,764
Fair Value, Measurements, Recurring | Corporate debt securities
Financial assets:
Financial assets, fair value	230,704	211,109
Fair Value, Measurements, Recurring | Equity warrant investment
Financial assets:
Financial assets, fair value	1,124	1,323
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Financial assets:
Financial assets, fair value	331,349	50,513
Financial liabilities:
Success payment liabilities	0	0
Total financial liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Money Market Funds [Member]
Financial assets:
Financial assets, fair value	331,349	50,513
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. Treasury Securities [Member]
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. government agency securities
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Corporate debt securities
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Equity warrant investment
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Financial assets:
Financial assets, fair value	584,426	619,574
Financial liabilities:
Success payment liabilities	0	0
Total financial liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Money Market Funds [Member]
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. Treasury Securities [Member]
Financial assets:
Financial assets, fair value	249,761	202,701
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities
Financial assets:
Financial assets, fair value	103,961	205,764
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities
Financial assets:
Financial assets, fair value	230,704	211,109
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Equity warrant investment
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Financial assets:
Financial assets, fair value	1,124	1,323
Financial liabilities:
Success payment liabilities	25,116	5,773
Total financial liabilities	25,116	5,773
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Money Market Funds [Member]
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | U.S. Treasury Securities [Member]
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | U.S. government agency securities
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Corporate debt securities
Financial assets:
Financial assets, fair value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Equity warrant investment
Financial assets:
Financial assets, fair value	$ 1,124	$ 1,323

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Estimated Fair Value of Success Payment Liability Assumptions (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)	Dec. 31, 2020USD ($)
Fred Hutch
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of common stock Series A convertible preferred stock	$ 14.80	$ 9.07
Fred Hutch | Risk-Free Interest Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Risk-free interest rate minimum	0.07	0.10
Risk-free interest rate maximum	2.24	1.52
Fred Hutch | Expected Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected volatility	75.00%	80.00%
Fred Hutch | Expected Term | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	8 months 15 days	1 year
Fred Hutch | Expected Term | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	6 years 2 months 19 days	6 years 11 months 19 days
Stanford
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of common stock Series A convertible preferred stock	$ 14.80	$ 9.07
Stanford | Risk-Free Interest Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Risk-free interest rate minimum	0.07	0.10
Risk-free interest rate maximum	2.24	1.53
Stanford | Expected Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected volatility	75.00%	80.00%
Stanford | Expected Term | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	8 months 15 days	1 year
Stanford | Expected Term | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	8 years	8 years 9 months

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Changes in Estimated Fair Value of Financial Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Equity warrant investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December 31, 2020	$ 1,323
Changes in fair value	(199)	[1]
Balance at September 30, 2021	1,124
Fair Value, Inputs, Level 3 | Success Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December 31, 2020	5,773
Change in fair value	19,343	[1]
Balance at September 30, 2021	$ 25,116
[1]	The change in fair value associated with the equity warrant investment is recorded in other income (expense), net and the change in fair value associated with success payment liabilities is recorded in research and development expense.

Leases - Summary of Operating L

Leases - Summary of Operating Lease Commitments Including Expected Lease Incentives to be Received (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Leases [Abstract]
2021 (remaining three months)	$ 2,419
2022	10,698
2023	11,018
2024	11,347
2025	11,859
Thereafter	60,302
Total undiscounted lease payments	107,643
Less: imputed interest	(33,422)
Less: tenant improvement allowances	(6,993)
Total operating lease liabilities	67,228
Operating Lease, Liability [Abstract]
Operating lease liabilities, non-current	65,684	$ 50,957
Total	67,228
Accrued Liabilities and Other Current Liabilities [Member]
Operating Lease, Liability [Abstract]
Short-term portion of lease liabilities (included in accrued liabilities and other current liabilities)	$ 1,544

Leases - Additional Information

Leases - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)ft²	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)ft²	Sep. 30, 2020USD ($)	May 31, 2021ft²
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 2.4	$ 2.9	$ 7.1	$ 8.3
Variable lease cost	1.4	$ 0.5	$ 3.7	$ 1.5
Non-related Party [Member]
Lessee, Lease, Description [Line Items]
Rentable area | ft²					11,000
Monthly fixed rental payment proceeds	$ 0.1
Sonoma [Member]
Lessee, Lease, Description [Line Items]
Rentable area | ft²	18,000		18,000
Monthly fixed rental payment proceeds	$ 0.1
Tenant improvements	$ 4.6		$ 4.6

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 17, 2021	Dec. 31, 2020
Convertible preferred stock, shares issued		0			194,474,000
Proceeds from issuance of convertible preferred stock, net of issuance costs			$ 492,467
Repurchase of convertible preferred stock			$ (3,582)
Outstanding shares of convertible preferred stock converted into shares of common stock				194,474,431
Outstanding convertible preferred stock (in shares)		0			194,474,000
Series C Convertible Preferred Stock
Convertible preferred stock, shares issued	42,905,042
Share price	$ 11.49
Proceeds from issuance of convertible preferred stock, net of issuance costs	$ 492,500
Net of issuance costs	$ 500
Series A Convertible Preferred Stock
Repurchase of convertible preferred stock, shares	546,806
Repurchase of convertible preferred stock	$ 4,200

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Jun. 30, 2021	Mar. 31, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Class Of Stock [Line Items]
Preferred and common shares authorized	510,000,000
Common stock, shares authorized				500,000,000		264,905,000
Preferred stock, par value	$ 0.0001			$ 0.0001		$ 0.0001
Preferred stock, shares authorized	10,000,000			10,000,000		0
Preferred stock, shares issued				0		0
Preferred stock, shares outstanding				0		0
Payments for the repurchase of common stock					$ 11,806
Related Party
Class Of Stock [Line Items]
Repurchase of shares		2,032,166
Payments for the repurchase of common stock			$ 11,800
Restricted Stock [Member]
Class Of Stock [Line Items]
Common stock outstanding				3,575,002		7,562,503
Common Stock
Class Of Stock [Line Items]
Common stock, shares authorized	500,000,000
Common Stock, shares, outstanding				239,789,419		15,569,788
Repurchase of shares		2,032,166			2,032,000
Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	10,000,000

Stock-based Compensation - Addi

Stock-based Compensation - Additional Infomation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2021	Jun. 16, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares, issued	239,789,000	239,789,000		239,789,000		15,570,000
Weighted average grant date fair value				$ 6.55	$ 3.21
Share Based Compensation Expense		$ 13,042	$ 9,748	$ 41,023	$ 20,290
Total stock-based compensation cost related to unvested awards not yet recognized	$ 122,500	$ 122,500		$ 122,500
Expected weighted-average period compensation cost to be recognized				2 years 5 months 15 days
IPO [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Payment Arrangement, Accelerated Cost				$ 2,600
2021 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period				4 years
Common shares reserved for future issuance	24,900,000	24,900,000		24,900,000				26,662,087
Percentage of Common shares reserved for future issuance increase				5.00%
2021 Plan | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period				10 years
2021 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares, issued	0	0		0
Common shares reserved for future issuance							2,470,000
Percentage of Common shares reserved for future issuance increase						1.00%
Share-based compensation arrangement by share-based payment award, other share increase (decrease)				4,940,000
Percentage of Purchase of shares at fair value	85.00%
2021 ESPP | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum offering period	27 months
Purchase of common stock at discount	15.00%	15.00%		15.00%

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Stock-Based Compensation Expense by Classification (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 13,042	$ 9,748	$ 41,023	$ 20,290
Research And Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,673	1,965	12,615	9,033
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 10,369	$ 7,783	$ 28,408	$ 11,257

Stock-based Compensation - Summ

Stock-based Compensation - Summary of RSA Activity (Details) - Unvested RSAs	9 Months Ended
	Sep. 30, 2021$ / sharesshares
RSA's
Unvested shares as of December 31, 2020 | shares	7,562,503
Vested | shares	(3,968,751)
Forfeited | shares	18,750
Unvested shares as of September 30, 2021 | shares	3,575,002
Weighted-Average Grant Date Fair Value per Share
Unvested shares as of December 31, 2020 | $ / shares	$ 0.0001
Vested | $ / shares	0.0001
Forfeited | $ / shares	0.0001
Unvested shares as of September 30, 2021 | $ / shares	$ 0.0001

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2021
Stock Options
Stock Options Outstanding, Beginning Balance	34,413,889
Stock Options, Granted	12,521,501
Stock Options, Exercised	(776,449)
Stock Options, Canceled or forfeited	(2,597,874)
Stock Options Outstanding, Ending Balance	43,561,067
Stock Options Exercisable	24,397,665
Weighted-Average Exercise Price per Share
Weighted-Average Exercise Price per Share Outstanding, Beginning Balance	$ 3.33
Weighted-Average Exercise Price per Share, Granted	9
Weighted-Average Exercise Price per Share, Exercised	2.63
Weighted-Average Exercise Price per Share, Cancelled or forfeited	4
Weighted-Average Exercise Price per Share Outstanding, Ending Balance	4.93
Weighted-Average Exercise Price Per Share Exercisable	$ 3.29
Weighted-Average Remaining Contractual Life (years)
Weighted-Average Remaining Contractual Life (years), Outstanding	7 years 11 months 19 days
Weighted-Average Remaining Contractual Life (years), Exercisable	7 years 2 months 1 day
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding	$ 432,916
Aggregate Intrinsic Value, Exercisable	$ 280,840

Stock-based Compensation - Sc_2

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model for Estimating Fair Value of Stock Options Granted (Details)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Risk-free interest rate	0.79%	0.99%
Expected volatility	79.00%	75.00%
Expected term (in years)	6 years 1 month 6 days	6 years 3 months
Expected dividend yield	0.00%	0.00%

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potentially Dilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of dilutive net loss per share	47,136,069	236,319,363	47,136,069	236,319,363
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of dilutive net loss per share	0	194,474,431	0	194,474,431
Unvested RSAs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of dilutive net loss per share	3,575,002	9,486,462,000	3,575,002	9,486,462,000
Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of dilutive net loss per share	43,561,067	32,358,470	43,561,067	32,358,470

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Operating lease liabilities, non-current		$ 65,684		$ 65,684		$ 50,957
Deferred revenue		5,581		5,581		6,095
Deferred revenue, net of current portion		81,883		81,883		89,066
Common Stock [Member]
Related Party Transaction [Line Items]
Repurchase of shares	2,032,166				2,032,000
Accrued Liabilities and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Operating lease liabilities, current portion		1,544		1,544
Series A Convertible Preferred Stock
Related Party Transaction [Line Items]
Repurchase of convertible preferred stock, shares	546,806
GSK Agreement [Member]
Related Party Transaction [Line Items]
Deferred revenue		5,600		5,600		6,100
Deferred revenue, net of current portion		81,900		81,900		$ 89,100
Revenue recognized		$ 2,600	$ 1,100	$ 7,700	$ 5,400
GSK Agreement [Member] | Minimum
Related Party Transaction [Line Items]
Ownership percentage		10.00%		10.00%
Sonoma [Member]
Related Party Transaction [Line Items]
Tenant improvements		$ 4,600		$ 4,600
Operating lease liabilities, non-current		4,100		4,100
Sonoma [Member] | Other Operating Income, Net
Related Party Transaction [Line Items]
Operating lease, lease income		500		1,200
Sonoma [Member] | Accrued Liabilities and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Operating lease liabilities, current portion		$ 500		$ 500

Asset Acquisition - Additional

Asset Acquisition - Additional Information (Details) - USD ($) shares in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Stock issued during period value acquisitions					$ 4,000,000
Immulus, Inc [Member]
Business Acquisition [Line Items]
Percentage of outstanding equity acquired	100.00%
Payments to acquire businesses, gross	$ 3,500,000
Business acquisition, transaction costs	500,000
Stock issued during period value acquisitions	$ 4,000,000
Stock issued during period, shares, acquisitions	688,463
Total consideration	$ 8,000,000
Milestone payments expense incurred		$ 0	$ 0	$ 0	$ 0
Immulus, Inc [Member] | Maximum [Member]
Business Acquisition [Line Items]
Milestone payments payable	$ 37,000,000

Asset Acquisition - Summary of

Asset Acquisition - Summary of Fair Value of Assets Acquired (Details) - Immulus, Inc [Member] $ in Thousands	May 31, 2020USD ($)
Business Acquisition [Line Items]
Other assets	$ 487
In-process research and development (IPR&D)	7,528
Total assets acquired	$ 8,015