Annexon (ANNX) 10-K 25 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 15, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	ANNX
Entity Registrant Name	ANNEXON, INC.
Entity Central Index Key	0001528115
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Title of 12(b) Security	Common stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-39402
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-5414423
Entity Address, Address Line One	180 Kimball Way
Entity Address, Address Line Two	Suite 200
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	822-5500
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		38,157,618
Entity Public Float			$ 0
ICFR Auditor Attestation Flag	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the Annual Meeting of Stockholders, which will be filed with the Securities and Exchange Commission within 120 days after the end of the Registrant’s fiscal year ended December 31, 2020, are incorporated by reference into Part III of this Report.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 268,565	$ 43,931
Short-term investments	82,641
Prepaid expenses and other current assets	2,805	1,475
Total current assets	354,011	45,406
Property and equipment	1,935	2,138
Other long-term assets		2,354
Total assets	355,946	49,898
Current liabilities:
Accounts payable	3,734	2,371
Accrued liabilities	6,497	2,194
Deferred rent, current	391	366
Total current liabilities	10,622	4,931
Deferred rent	1,046	1,437
Total liabilities	11,668	6,368
Commitments and contingencies (Note 6)
Stockholders’ Equity (Deficit):
Preferred stock, $0.001 par value; 5,000,000 and zero shares authorized as of December 31, 2020 and 2019, respectively; zero issued and outstanding as of December 31, 2020 and 2019, respectively
Common stock, $0.001 par value; 300,000,000 and 150,000,000 shares authorized as of December 31, 2020 and 2019, respectively; 38,157,618 and 433,749 shares issued and outstanding as of December 31, 2020 and 2019, respectively	38	4
Additional paid-in capital	510,309	2,202
Accumulated other comprehensive loss	(77)	(80)
Accumulated deficit	(165,992)	(102,580)
Total stockholders’ equity (deficit)	344,278	(100,454)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	355,946	49,898
Redeemable Convertible Preferred Stock [Member]
Current liabilities:
Redeemable convertible preferred stock, $0.001 par value; zero and 119,155,472 shares authorized as of December 31, 2020 and 2019, respectively; zero and 111,748,065 shares issued and outstanding as of December 31, 2020 and 2019, respectively; aggregate liquidation preference of zero and $137,814 as of December 31, 2020 and 2019, respectively	$ 0	$ 143,984

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 0.001	$ 0.001
Common stock, shares authorized	300,000,000	150,000,000
Common stock, shares issued	38,157,618	433,749
Common stock, shares outstanding	38,157,618	433,749
Redeemable Convertible Preferred Stock [Member]
Temporary equity, par value per share	$ 0.001	$ 0.001
Temporary equity, shares authorized	0	119,155,472
Temporary equity, shares issued	0	111,748,065
Temporary equity, shares outstanding	0	111,748,065
Temporary equity, liquidation preference	$ 0	$ 137,814

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 49,271	$ 24,524
General and administrative	14,198	7,994
Total operating expenses	63,469	32,518
Loss from operations	(63,469)	(32,518)
Loss on remeasurement of redeemable convertible preferred stock liability		(5,670)
Other income, net	57	1,009
Net loss before taxes	(63,412)	(37,179)
Provision for income taxes		4
Net loss	(63,412)	(37,183)
Accretion on redeemable convertible preferred stock	(705)	(1,095)
Deemed dividend – beneficial conversion feature on redeemable convertible preferred stock	(6,219)
Net loss attributable to common stockholders	$ (70,336)	$ (38,278)
Net loss per share attributable to common stockholders, basic and diluted	$ (4.15)	$ (88.30)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	16,962,398	433,493

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (63,412,000)	$ (37,183,000)
Other comprehensive gain (loss):
Foreign currency translation adjustment	9,000	(14,000)
Unrealized loss on available for sale securities	(6,000)
Comprehensive loss	$ (63,409,000)	$ (37,197,000)

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	Total	Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member]	Series D Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance, Shares at Dec. 31, 2018		89,525,848
Beginning Balance, Values at Dec. 31, 2018		$ 102,082,000
Beginning Balance, Shares at Dec. 31, 2018					432,309
Beginning Balance, Values at Dec. 31, 2018	$ (64,202,000)				$ 4,000	$ 1,257,000	$ (66,000)	$ (65,397,000)
Issuance of redeemable convertible preferred stock, net of issuance costs and value of redeemable convertible preferred stock liability, Shares			22,222,217
Issuance of redeemable convertible preferred stock, net of issuance costs and value of redeemable convertible preferred stock liability, Values			$ 40,807,000
Accretion on redeemable convertible preferred stock		$ 1,095,000
Accretion on redeemable convertible preferred stock	(1,095,000)					(1,095,000)
Stock option exercises, Shares					1,440
Stock option exercises, Values	3,000					3,000
Stock-based compensation	2,037,000					2,037,000
Foreign currency translation adjustment	(14,000)						(14,000)
Net loss	(37,183,000)							(37,183,000)
Ending Balance, Shares at Dec. 31, 2019		111,748,065	55,555,546
Ending Balance, Values at Dec. 31, 2019		$ 143,984,000	$ 10,810,000
Ending Balance, Shares at Dec. 31, 2019					433,749
Ending Balance, Values at Dec. 31, 2019	(100,454,000)				$ 4,000	2,202,000	(80,000)	(102,580,000)
Issuance of redeemable convertible preferred stock, net of issuance costs and value of redeemable convertible preferred stock liability, Shares				71,719,859
Issuance of redeemable convertible preferred stock, net of issuance costs and value of redeemable convertible preferred stock liability, Values				$ 96,807,000
Accretion on redeemable convertible preferred stock		705,000
Accretion on redeemable convertible preferred stock	(705,000)					(705,000)
Beneficial conversion feature on Series D redeemable convertible preferred stock		$ (6,297,000)
Beneficial conversion feature on Series D redeemable convertible preferred stock	6,297,000					6,297,000
Conversion of redeemable convertible preferred stock to common stock upon initial public offering, Shares		(183,467,924)
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	6,297,000	$ (241,418,000)
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	241,418,000				$ 21,000	241,397,000
Conversion of redeemable convertible preferred stock to common stock upon initial public offering, Shares					20,824,938
Deemed dividend of beneficial conversion feature on conversion of redeemable convertible preferred stock		$ 6,219,000
Deemed dividend of beneficial conversion feature on conversion of redeemable convertible preferred stock	(6,219,000)					(6,219,000)
Issuance of common stock in connection with initial public offering, net of issuance costs of $24,700, Shares					16,889,403
Issuance of common stock in connection with initial public offering, net of issuance costs of $24,700, Values	$ 262,440,000				$ 13,000	262,427,000
Stock option exercises, Shares	9,528				9,528
Stock option exercises, Values	$ 22,000					22,000
Stock-based compensation	4,888,000					4,888,000
Foreign currency translation adjustment	9,000						9,000
Unrealized loss on available-for-sale securities	(6,000)						(6,000)
Net loss	(63,412,000)							(63,412,000)
Ending Balance, Shares at Dec. 31, 2020		0
Ending Balance, Values at Dec. 31, 2020		$ 0
Ending Balance, Shares at Dec. 31, 2020					38,157,618
Ending Balance, Values at Dec. 31, 2020	$ 344,278,000				$ 38,000	$ 510,309,000	$ (77,000)	$ (165,992,000)

Consolidated Statements of Re_2

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Payments of stock issuance costs	$ 1,605
Series C Redeemable Convertible Preferred Stock [Member]
Payments of stock issuance costs	3
Redeemable convertible preferred stock, $0.001 par value; zero and 119,155,472 shares authorized as of December 31, 2020 and 2019, respectively; zero and 111,748,065 shares issued and outstanding as of December 31, 2020 and 2019, respectively; aggregate liquidation preference of zero and $137,814 as of December 31, 2020 and 2019, respectively	$ 10,810

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating activities:
Net loss	$ (63,412)	$ (37,183)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	667	493
Accretion of discount on available-for sale securities	53
Stock-based compensation	4,888	2,037
Change in fair value of redeemable convertible preferred stock liability		5,670
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,330)	56
Other long-term assets	96
Accounts payable	1,513	931
Accrued liabilities	4,804	(20)
Deferred rent	(366)	(342)
Net cash used in operating activities	(53,087)	(28,358)
Investing activities:
Purchases of property and equipment	(464)	(267)
Purchase of available-for-sale securities	(82,700)
Net cash used in investing activities	(83,164)	(267)
Financing activities:
Proceeds from the exercise of common stock options	22	3
Proceeds from Paycheck Protection Program loan	500
Repayments of Paycheck Protection Program loan	(500)
Proceeds from issuance of redeemable convertible preferred stock (including $13,900 of proceeds from the related parties in 2020)	102,000	29,997
Payments of issuance costs related to redeemable convertible preferred stock	(5,193)
Proceeds from issuance of common stock upon initial public offering, net of underwriting discounts and commissions	267,021
Payments of offering costs related to initial public offering	(2,974)	(1,605)
Net cash provided by financing activities	360,876	28,395
Net increase (decrease) in cash and cash equivalents	224,625	(230)
Effect of exchange rate changes on cash and cash equivalents	9	(14)
Cash and cash equivalents at beginning of period	43,931	44,175
Cash and cash equivalents at end of period	268,565	43,931
Supplemental disclosures of cash flow information:
Income taxes paid		2
Non-cash investing and financing activities:
Reclassification of redeemable convertible preferred stock to common stock upon initial public offering	241,418
Reclassification of redeemable convertible preferred stock liability to redeemable convertible preferred stock		10,810
Accretion on redeemable convertible preferred stock	705	1,095
Deferred offering costs included in accounts payable and accrued liabilities		651
Purchase of property and equipment included in accounts payable		$ 19
Beneficial conversion feature recognized upon issuance of redeemable convertible preferred stock	6,297
Deemed dividend arising from conversion of beneficial conversion feature	$ 6,219

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Proceeds from issuance of redeemable convertible preferred stock	$ 102,000
Related Parties [Member]
Proceeds from issuance of redeemable convertible preferred stock	$ 13,900

Organization

Organization	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Organization	1. Organization Annexon, Inc., or the Company, is a clinical-stage biopharmaceutical company targeting C1q and initiating molecules of the classical complement pathway to develop transformative therapies for autoimmune and neurodegenerative disorders of the body, brain and eye. The Company is located in South San Francisco, California and was incorporated in Delaware in March 2011. The Company’s wholly-owned subsidiary, Annexon Biosciences Australia Pty Ltd, or the Subsidiary, is a proprietary limited company incorporated in 2016 and domiciled in Australia. The Subsidiary is also engaged in research and development activities in support of its parent company. Initial Public Offering On July 23 14,750,000 2,139,403 262.4 Reverse Stock Split On July 17, 2020, the Company’s board of directors approved an amendment to the Company’s certificate of incorporation to effect a reverse split of shares of the Company’s common stock on a one-for-8.81 basis, or the Reverse Stock Split. The number of authorized shares and the par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the Reverse Stock Split. In connection with the Reverse Stock Split, the conversion ratio for the Company’s outstanding redeemable convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. All references to common stock and options to purchase common stock share data, per share data and related information contained in the consolidated financial statements have been adjusted to reflect the effect of the Reverse Stock Split. Liquidity Since inception, the Company has been involved primarily in performing research and development activities, conducting clinical trials, hiring personnel, and raising capital to support and expand these activities. The Company has experienced losses and negative cash flows from operations since its inception and, as of December 31, 2020, had an accumulated deficit of $166.0 million and cash and cash equivalents and short-term investments of $351.2 million. The Company has historically funded its operations through the issuance of shares of its redeemable convertible preferred stock and common stock. Based on projected activities, management projects that cash on hand is sufficient to support operations for at least the next 12 months following issuance of these consolidated financial statements. Management expects to continue to incur losses and negative cash flows from operations for at least the next several years.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation and Significant Accounting Policies	2 . Basis of Presentation Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including but not limited to the fair value of common stock, redeemable convertible preferred stock, redeemable convertible preferred stock liability, stock options, income taxes, clinical trial accruals and stock-based compensation. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Principles The consolidated financial statements include the operations of Annexon, Inc. and its wholly owned subsidiary and include the results of operations and cash flows of these entities. All intercompany balances and transactions have been eliminated in consolidation. Segments The Company’s chief operating decision maker is its Chief Executive Officer. The Chief Executive Officer reviews financial information on an aggregate basis for the purposes of evaluating financial performance and allocating the Company’s resources. Accordingly, the Company has determined that it operates in one segment. Cash and Cash Equivalents The considers Short-Term Investments Short-term investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. The Company determines the appropriate classification of its investments in debt securities at the time of purchase. Available-for-sale securities with original maturities beyond three months at the date of purchase are classified as current based on their availability for use in current operations. Unrealized gains and losses are excluded from earnings and are reported as a component of other comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Impairment assessments are made at the individual security level each reporting period. When the fair value of an available-for-sale security is less than its cost at the balance sheet date, a determination is made as to whether the impairment is other-than-temporary and, if it is other-than-temporary, an impairment loss is recognized in other income, net, equal to the difference between the investment’s amortized cost and fair value at such date. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net. Property and Equipment, Net Property and equipment are carried at cost less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations in the period realized. The useful Laboratory equipment 5 years Office and computer equipment 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. When indications of impairment are present and the estimated undiscounted future cash flows from the use of these assets is less than the assets’ carrying value, the related assets will be written down to fair value. There were no impairments of the Company’s long-lived assets for the periods presented. Deferred Offering Costs Deferred offering costs, consisting of legal, accounting, audit and filing fees relating to an initial public offering, are capitalized. As of December 31, 2019, $2.3 million of deferred offering costs were capitalized, which are included in other long-term assets in the accompanying consolidated balance sheets. In July 2020, upon the closing of the IPO, all deferred offering costs were offset against the Company’s IPO proceeds within additional paid-in capital. Redeemable Convertible Preferred Stock Liability The obligation to issue additional shares of the Company’s Series C redeemable convertible preferred stock at a future date was determined to be a freestanding financial instrument that should be accounted for as a liability. At initial recognition, the Company recorded the redeemable convertible preferred stock liability on the balance sheet at its estimated fair value. The liability was subject to remeasurement at each balance sheet date, with changes in fair value recognized as gain (loss) on remeasurement of redeemable convertible preferred stock liability on the consolidated statement of operations. Upon settlement of the redeemable convertible preferred stock liability in August 2019, the Company remeasured the liability and reclassified the final value associated with the redeemable convertible preferred stock liability to the carrying value of the Series C redeemable convertible preferred stock. Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Valuation allowances are established when necessary to reduce deferred taxes to the amounts expected to be realized. The examination Commitments and Contingencies Liabilities for loss contingencies arising from claims, assessments, litigation, fines, and penalties and other sources are recorded when it is probable that a liability has been incurred and the amount can be reasonably estimated. Legal costs incurred in connection with loss contingencies are expensed as incurred. Foreign Currencies The Dollar Gains Research and Development Expense Research and development expenses consist primarily of direct and indirect costs incurred for the development of the Company’s product candidates. Direct expenses include (i) preclinical and clinical outside service costs associated with discovery, preclinical and clinical testing of the Company’s product candidates; (ii) professional services agreements with third-party contract organizations, investigative clinical trial sites and consultants that conduct research and development activities on the Company’s behalf; (iii) contract manufacturing costs to produce clinical trial materials; and (iv) laboratory supplies and materials. Indirect expenses include (A) compensation and personnel- related expenses (including stock-based compensation), (B) allocated expenses for facilities and depreciation; and (C) other indirect costs. Research and development costs are expensed as incurred. Payments made to third parties are under agreements that are generally cancelable by the Company. Advance payments for research and development activities are deferred as prepaid expenses. The prepaid amounts are expensed as the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the period over which services will be performed and the level of effort to be expended in each period. These estimates are based on the Company’s communications with the third-party service providers and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies significantly from the estimate, the Company will adjust Stock-Based Compensation The The Company grants certain employees performance-based stock options. For awards that include performance conditions, no compensation cost is recognized until the performance goals are probable of being met, at which time the cumulative compensation expense from the service inception date would be recognized. Stock-based compensation costs are based on the fair value of the underlying option calculated using the Black-Scholes option pricing model and recognized as expense on a straight-line basis (for all but performance based awards for which the accelerated method is used) over the requisite service period, which is the vesting period. Determining the appropriate fair value model and related assumptions requires judgment, including estimating the fair value of the underlying common stock, expected term, expected stock price volatility, risk-free interest rate and dividend yield. The Company accounts for forfeitures Accounting for Non-Recurring Grant Income Non-recurring grant income is recognized when the research and development activities have been undertaken and the Company has completed its assessment of whether such activities meet the relevant qualifying criteria. Grants received from government and other agencies in advance of the specific research and development costs to which they relate are deferred and recognized in the consolidated statement of operations in the period they are earned and when the related research and development costs are incurred. Non-recurring grant income recognized in other income, net was $190,000 for the year ended December 31, 2019. No non-recurring grant income was recognized during 2020. Net Loss Per Share Attributable Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. As the Company was in a loss position for all periods presented, basic net loss per share attributable to common stockholders is the same as diluted net loss per share attributable to common stockholders because the effects of potentially dilutive securities are antidilutive. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. The Company’s cash and Management Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 or the JOBS Act. Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (Topic 718) In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement (Topic 820) In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740). The guidance eliminates certain exceptions for recognizing deferred taxes for investments, performing intraperiod allocation and calculating income taxes in interim periods. This guidance also includes guidance to reduce complexity in certain areas, including recognizing deferred taxes for tax goodwill and allocating taxes to members of a consolidated group. ASU No. 2019-12 is effective for annual and interim periods in fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company early adopted this standard on January 1, 2020. The adoption of this standard did not have a material impact on the financial statements. Recently In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), , Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments - Credit Losses Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 825), Codification Improvements to Financial Instruments is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies. In August 2018, the FASB issued ASU No. 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract (Topic 350)

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3 . Fair Value Measurements The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines fair value based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: • Level 1 Inputs: Unadjusted quoted prices in active markets for identical assets or liabilities accessible to the reporting entity at the measurement date. • Level Other than quoted prices included in Level 1 inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability. • Level Unobservable inputs for the asset or liability used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at measurement date. On a recurring basis, the Company measures certain financial assets and liabilities at fair value. The following tables summarize the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): December 31, 2020 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 267,968 $ — $ — $ 267,968 Short-term investments: Commercial paper Level 2 59,930 — — 59,930 Corporate bonds Level 2 22,717 1 (7 ) 22,711 Total assets $ 350,615 $ 1 $ (7 ) $ 350,609 December 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 43,621 $ — $ — $ 43,621 Total assets $ 43,621 $ — $ — $ 43,621 All of the investments held as of December 31, 2020 had maturities of less than two years. For the years ended December 31, 2020 and 2019, the Company recognized no material realized gains or losses on financial instruments. In August 2019, the redeemable convertible preferred stock liability was settled upon the completion of the second closing of the Company’s Series C redeemable convertible preferred stock financing. In light of the Company’s progress towards an IPO, the liability was remeasured at the settlement date using a probability-weighted expected return method, or PWERM, whereby the Company’s total equity value was estimated under various exit scenarios and allocated to the Company’s different classes of equity. The PWERM included two scenarios, IPO or staying private, that considered an estimate of the timing of each scenario and were weighted based on the Company’s estimate of the probability of each event occurring. The equity value under the IPO scenario was based on recent IPO values of comparable companies and weighted 50%. The equity value under the staying private scenario was based on the recent Series C redeemable convertible preferred stock financing and was weighted 50%. The liability was remeasured to its fair value of $10.8 million upon settlement and the carrying value of the liability was reclassified to the carrying value of the Series C redeemable convertible preferred stock. The Company recorded a loss of $5.7 million in the consolidated statements of operations for the year ended December 31, 2019, for the change in the fair value of the liability. The changes in the carrying value of the liability were as follows (in thousands): Fair value as of December 31, 2018 $ 5,140 Change in fair value 5,670 Reclassification to redeemable convertible preferred stock upon settlement (10,810 ) Fair value as of December 31, 2019 $ —

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2020
Disclosure Of Balance Sheet Components [Abstract]
Balance Sheet Components	4 . Balance Sheet Components Prepaid Expenses and Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2020 2019 Prepaid insurance $ 1,236 $ 16 Prepaid research and development costs 1,039 1,086 Other prepaid expenses 527 294 Other receivables 3 79 Total prepaid expenses and other current assets $ 2,805 $ 1,475 Property and Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Leasehold improvements $ 3,061 $ 2,772 Laboratory equipment 555 456 Furniture and fixtures 263 187 Computer equipment and software 27 27 Total property and equipment, gross $ 3,906 $ 3,442 Less: accumulated depreciation (1,971 ) (1,304 ) Total property and equipment, net $ 1,935 $ 2,138 Total depreciation expense recognized for the years ended December 31, 2020 and 2019 was $667,000 and $493,000, respectively. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2020 2019 Accrued research and development expenses $ 3,260 $ 459 Accrued compensation 2,543 926 Accrued professional services 524 733 Other accrued expenses 170 76 Total accrued liabilities $ 6,497 $ 2,194

Promissory Note

Promissory Note	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Promissory Note	5 . Promissory Note Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, the Company obtained a $0.5 million promissory note from the U.S. Small Business Administration on April 21, 2020, which was repaid in full on May 1, 2020.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6 . Commitments Leases The Company leases its offices and laboratory in South San Francisco, California under a 7-year noncancelable lease agreement that ends in June 2024 with a 5-year renewal option. In December 2020, the Company entered into an agreement to lease office and laboratory space in Brisbane, California, or the Brisbane Lease, with an affiliate of its current landlord. The term of the Brisbane Lease is expected to begin in November 2021 and end in October 2031, with an option by the Company to extend for an additional ten years. Concurrent with the execution of the Brisbane Lease, the Company entered into an agreement to terminate its existing lease in South San Francisco, California effective upon the commencement of the Brisbane Lease. The base rent will be abated for three months and the landlord will provide the Company with a tenant improvement allowance of up to $10.8 million. In connection with the Brisbane Lease, the Company is required to maintain a letter of credit for the benefit of the landlord in the amount of $1.0 million, which was delivered in January 2021. Rent expense is recognized on a straight-line basis over the non-cancelable term of the lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability on the accompanying consolidated balance sheet. Total rent expense was Future minimum lease payments under noncancelable operating leases as of December 31, 2020 were as follows (in thousands): 2021 $ 618 2022 4,189 2023 4,756 2024 5,035 2025 and thereafter 38,332 Total $ 52,930 License and In November 2011, the Company entered into an exclusive licensing agreement , or the Stanford Agreement, with The Board of Trustees of the Leland Stanford Junior University, or Stanford, whereby the Company was granted an exclusive, worldwide, royalty-bearing, sublicensable license, under certain patent rights, or the Licensed Patents, to make, use, offer for sale, sell, import and otherwise commercialize products covered by the Licensed Patents for human or animal diseases, disorders or conditions. Under the Stanford Agreement, the Company made an upfront payment and is obligated to pay Stanford annual license maintenance fees, potential future milestone payments totaling up to $500,000, and royalty payments at a rate equal to a low single-digit percentage of worldwide net sales of licensed products. In December 2020, the Company achieved one of the specified development milestones relating to the commencement of the ANX005 Phase 2/3 GBS study, pursuant to the Stanford Agreement. As such, the milestone payment of $0.1 million was expensed to research and development for the year ended December 31, 2020. In December 2016, the Company entered into a Sponsored Research Agreement with a not-for-profit entity to perform research on multiple sclerosis. The Sponsored Research Agreement was amended in March 2019. Under the terms of the Sponsored Research Agreement, as amended, the Company may receive up to $651,000 No Guarantees and In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of December 31, 2020, the Company did not have any material indemnification claims or guarantees that were probable or reasonably possible and consequently has not recorded related liabilities.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit)	12 Months Ended
	Dec. 31, 2020
Temporary Equity [Abstract]
Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit)	7 . Redeemable Convertible Equity ( Redeemable As of December 31, 2019, redeemable convertible preferred stock consisted of the following: Shares Authorized Shares Outstanding Net Carrying Value (in thousands) Liquidation Preference (in thousands) Series A 1,015,434 1,015,434 $ 1,000 $ 1,000 Series A-1 16,398,995 16,398,995 17,144 17,219 Series B 38,778,091 38,778,090 44,505 44,595 Series C 62,962,952 55,555,546 81,335 75,000 Total 119,155,472 111,748,065 $ 143,984 $ 137,814 In June 2020, the Company completed its financing of Series D redeemable convertible preferred stock, which resulted in the Company issuing 71,719,859 shares of Series D redeemable convertible preferred stock, including 9,773,587 shares issued to related parties who were 10% or greater holders of the Company, at a price per share of $1.4222 for aggregate proceeds of $96.8 million, net of issuance costs of $5.2 million. The shares of Series D redeemable convertible preferred stock were convertible at any time, at the holders’ option, into shares of common stock. In connection with the issuance and sale of the Series D redeemable convertible preferred stock, the Company recognized a beneficial conversion feature of approximately $6.3 million. A beneficial conversion feature represents the intrinsic value of the conversion feature, as determined by comparing the effective conversion price at the commitment date with the estimated fair value of the Company’s common stock. The beneficial conversion feature was recorded in additional paid-in capital on the issuance date resulting in a discount to the carrying value of the Series D redeemable convertible preferred stock. In July 2020, immediately prior to the completion of the IPO (see Note 1), all outstanding shares of redeemable convertible preferred stock were automatically converted into 20,824,938 shares of common stock. Upon conversion of the redeemable convertible preferred stock, the unamortized balance of $6.2 million of the discount from the beneficial conversion feature was recorded as a deemed dividend to the preferred stockholders resulting in an increase to the carrying value of redeemable convertible preferred stock. Upon conversion into common stock, the carrying value of the redeemable convertible preferred stock of $241.4 million was reclassified to equity. Common Stock The holders of the Company’s common stock have one vote for each share of common stock. Common stockholders are entitled to dividends when, as, and if declared by the Board of Directors. The holders have no preemptive or other subscription rights and there are no redemption or sinking fund provisions with respect to such shares. As of December 31, 2020, no dividends had been declared by the Board of Directors. The Company reserved the following shares of common stock for issuance as follows: December 31, 2020 2019 Redeemable convertible preferred stock outstanding on an as-converted basis — 12,684,214 Options issued and outstanding 3,909,873 2,007,222 Options available for future grant 2,707,947 454,110 Reserved for employee stock purchase plan 360,086 — Total common stock reserved 6,977,906 15,145,546

Equity Incentive Plan

Equity Incentive Plan	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Incentive Plan	8. Equity Incentive In July 2020, the Company’s board of directors and stockholders adopted and approved the 2020 Incentive Award Plan, or the 2020 Plan, and the Employee Stock Purchase Plan, or the ESPP, which became effective in connection with the IPO. The Company reserved 3,600,868 shares of common stock for future issuance under the 2020 Plan and 360,086 shares under the ESPP, respectively. The Company may not grant any additional awards under the 2011 Equity Incentive Plan, or the 2011 Plan. The 2011 Plan will continue to govern outstanding equity awards granted thereunder. 2020 Equity Incentive Plan The number of shares of common stock reserved for issuance under the 2020 Plan automatically increase on the first day of January, commencing on January 1, 2021 and ending in 2030, in an amount equal to 4% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors. Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For the Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. As of December 31, 2020, there were 2,707,947 Stock option activity under the 2011 Plan and the 2020 Plan was as follows: Shares Available for Grant Number of Shares Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances as of December 31, 2019 454,110 2,007,222 $ 4.91 8.26 $ 21,623 Additional shares authorized 4,166,016 — Stock options granted (1,914,819 ) 1,914,819 $ 16.88 Stock options exercised — (9,528 ) $ 2.33 Stock options cancelled 2,640 (2,640 ) $ 4.67 Balances as of December 31, 2020 2,707,947 3,909,873 $ 10.78 8.40 $ 56,128 Exercisable as of December 31, 2020 1,365,487 $ 4.96 6.92 $ 27,346 The total intrinsic value of options exercised during the years ended December 31, 2020 and 2019 was $181,000 The weighted-average grant date fair value of options granted to employees during the years ended December 31, 2020 and 2019 was $12.33 and $5.72 per share, respectively. Employee Stock Purchase Plan The ESPP enables eligible employees to purchase shares of the Company's common stock at the end of each offering period at a price equal to 85% of the fair market value of the shares on the first business day or the last business day of the offering period, whichever is lower. Eligible employees generally included all employees. Share purchases are funded through payroll deductions of at least 1%, and up to 15% of an employee’s eligible compensation for each payroll period. The number of shares reserved for issuance under the ESPP increase automatically on the first day of each fiscal year, beginning on January 1, 2021, by a number equal to the least of 360,086 shares, 1% of the shares of common stock outstanding on the last day of the immediately preceding fiscal year, or such number of shares determined by the Company’s board of directors. As of December 31, 2020, 360,086 shares were available for future purchase. The ESPP generally provides for six-month consecutive offering periods beginning on December 15, 2020. The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation. As such, stock-based compensation expense has been recorded for the year ended December 31, 2020. Stock-Based Compensation Expense The total stock-based compensation expense recognized was as follows (in thousands): Year Ended December 31, 2020 2019 Research and development $ 2,274 $ 713 General and administrative 2,614 1,324 Total stock-based compensation expense $ 4,888 $ 2,037 As of December 31, 2020, the total unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest was $25.8 million, which the Company expects to recognize over an estimated weighted-average period of 3.06 years. To determine the value of stock option awards for stock-based compensation purposes, the Company uses the Black-Scholes option pricing model and the assumptions discussed below. Fair Value of Common Stock— Historically, for all periods prior to the IPO in July 2020, fair values of the shares of common stock underlying the Company’s share-based awards were estimated on each grant date by the Company’s board of directors. The board of directors considered, among other things, valuations of the Company’s common stock which were prepared by an independent third- party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants 2013 Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. After the completion of the IPO in July 2020, the fair value of each share of underlying common stock will be based on the closing price of the Company’s common stock as reported on the date of grant on the Nasdaq Global Select Market. Expected Term —The Company’s expected term represents the period that the Company’s stock-based awards are expected to be outstanding. The Company’s historical share option exercise information is limited due to a lack of sufficient data points, and did not provide a reasonable basis upon which to estimate an expected term. The expected term for option grants is therefore determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). Expected Volatility —Because the Company does not have sufficient trading history for its common stock, the expected volatility was estimated based on the average volatility for comparable publicly traded life science companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on the similar size, stage in the life cycle, or area of specialty. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available and to align with the Company’s expected term. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the option. Dividend Yield —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. The fair value of each award issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Year Ended December 31, 2020 2019 Expected term (in years) 5.00-6.08 6.02-6.08 Expected volatility 85%-93% 72%-77% Risk-free interest rate 0.33%-1.45% 1.48%-2.61% Dividend yield — —

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes For financial reporting purposes, loss before provision for income taxes, includes the following components (in thousands): Year Ended December 31, 2020 2019 Domestic $ 63,406 $ 36,973 Foreign 6 206 Loss before income taxes $ 63,412 $ 37,179 For each of the years ended December 31, 2020 and 2019, the Company incurred insignificant amounts for an income tax provision. The U.S. federal and California deferred tax assets generated from the Company’s net operating losses have been fully reserved, as the Company believes it is more likely than not that the benefit will not be realized. Reconciliation of income tax computed at federal statutory rates to the reported provision for income taxes was as follows (in thousands): Year Ended December 31, 2020 2019 Tax provision at U.S. statutory rate $ (13,316 ) $ (7,808 ) State income taxes — 4 Stock-based compensation 426 112 R&D tax credits (1,031 ) (782 ) Change in valuation allowance 13,906 7,292 Redeemable convertible preferred stock liability — 1,191 Other 15 (5 ) Provision for income taxes $ — $ 4 Deferred The tax effects of temporary differences that give rise to significant portions of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2020 2019 Deferred Tax Assets: Net operating loss carryforwards $ 36,922 $ 24,917 Research and development credits 6,246 4,405 Other intangibles 5 6 Accruals and reserves 583 254 Stock-based compensation 995 393 Tenant improvement allowances 257 325 Total gross deferred tax assets 45,008 30,300 Less: valuation allowance (44,745 ) (29,961 ) Total deferred tax assets, net $ 263 $ 339 Deferred Tax Liabilities: Fixed assets $ (263 ) $ (339 ) Total gross deferred tax liabilities (263 ) (339 ) Net deferred tax assets $ — $ — As of December 31, 2020, the Company had $146.7 million of federal and $66.7 million of state net operating loss, or NOL, carryforwards available to offset future taxable income. Under the Tax Cuts and Jobs Act of 2017, or the Tax Act, NOLs generated after December 31, 2017 will be carried forward indefinitely with the yearly NOL utilization limited to 80 percent of taxable income. The Company had $103.6 million of such federal NOLs that do not expire. If not utilized, the federal carryforward losses generated prior to 2018 and the state carryforward losses will expire in various amounts beginning in 2031. As of December 31, 2020, the Company had $4.3 million of federal and $3.5 million of state credit carryforwards available to offset future taxable income. If not utilized, these credit carryforwards will expire in various amounts for federal purposes beginning in 2031. The state credits do not expire. In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some or all of the deferred tax assets will not be realized. Management believes that, based on available evidence, both positive and negative, it is more likely than not that the deferred tax assets will not be utilized; therefore, a full valuation allowance has been recorded. The Company’s valuation allowance increased by $14.8 million and $10.4 million for the years ended December 31, 2020 and 2019, respectively. The changes in the 2020 and 2019 valuation allowance were primarily due to the addition of the current year loss carryforwards and research and development credits. Utilization of the net operating loss carryforwards and credits may be subject to an annual limitation due to the ownership change limitations provided by the Internal Revenue Code of 1986, as amended, and similar state provisions. Any annual limitation may result in the expiration of net operating losses and credits before utilization. Uncertain Tax The Company has the following activity relating to the gross amount of unrecognized tax benefits (in thousands): Year Ended December 31, 2020 2019 Beginning balance $ 1,116 $ 783 Additions based on tax positions related to current year 460 333 Ending balance $ 1,576 $ 1,116 None of these uncertain tax positions will impact the Company’s effective tax rate if assessed. The Company’s policy is to classify interest and penalties associated with unrecognized tax benefits as income tax expense. The Company had no interest or penalty accruals associated with uncertain tax benefits in its consolidated balance sheet and consolidated statement of operations for the year ended December 31, 2020. The Company files income tax returns in California, Massachusetts, Pennsylvania and in Australia. The Company is not currently under examination by any major tax jurisdictions nor has it been in the past. The tax years 2011 through 2020 remain effectively open for examination by the Internal Revenue Service and most state tax authorities. Although it is reasonably possible that certain unrecognized tax benefits may increase or decrease within the next 12 months due to tax examination changes, settlement activities, expirations of statute of limitations, or the impact on recognition and measurement considerations related to the results of published tax cases or other similar activities, the Company does not anticipate any significant changes to unrecognized tax benefits over the next 12 months.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	10. Net Loss Per Share Attributable As the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share for all periods as the inclusion of all potential common securities outstanding would have been anti-dilutive . The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: Year Ended December 31, 2020 2019 Redeemable convertible preferred stock on an as-converted basis — 12,684,214 Stock options to purchase common stock 3,909,873 2,007,222 Shares subject to employee stock purchase plan 275 — Total 3,910,148 14,691,436

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including but not limited to the fair value of common stock, redeemable convertible preferred stock, redeemable convertible preferred stock liability, stock options, income taxes, clinical trial accruals and stock-based compensation. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Principles of Consolidation	Principles The consolidated financial statements include the operations of Annexon, Inc. and its wholly owned subsidiary and include the results of operations and cash flows of these entities. All intercompany balances and transactions have been eliminated in consolidation.
Segments	Segments The Company’s chief operating decision maker is its Chief Executive Officer. The Chief Executive Officer reviews financial information on an aggregate basis for the purposes of evaluating financial performance and allocating the Company’s resources. Accordingly, the Company has determined that it operates in one segment.
Cash and Cash Equivalents	Cash and Cash Equivalents The considers
Short-Term Investments	Short-Term Investments Short-term investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. The Company determines the appropriate classification of its investments in debt securities at the time of purchase. Available-for-sale securities with original maturities beyond three months at the date of purchase are classified as current based on their availability for use in current operations. Unrealized gains and losses are excluded from earnings and are reported as a component of other comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Impairment assessments are made at the individual security level each reporting period. When the fair value of an available-for-sale security is less than its cost at the balance sheet date, a determination is made as to whether the impairment is other-than-temporary and, if it is other-than-temporary, an impairment loss is recognized in other income, net, equal to the difference between the investment’s amortized cost and fair value at such date. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are carried at cost less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations in the period realized. The useful Laboratory equipment 5 years Office and computer equipment 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. When indications of impairment are present and the estimated undiscounted future cash flows from the use of these assets is less than the assets’ carrying value, the related assets will be written down to fair value. There were no impairments of the Company’s long-lived assets for the periods presented.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs, consisting of legal, accounting, audit and filing fees relating to an initial public offering, are capitalized. As of December 31, 2019, $2.3 million of deferred offering costs were capitalized, which are included in other long-term assets in the accompanying consolidated balance sheets. In July 2020, upon the closing of the IPO, all deferred offering costs were offset against the Company’s IPO proceeds within additional paid-in capital.
Redeemable Convertible Preferred Stock Liability	Redeemable Convertible Preferred Stock Liability The obligation to issue additional shares of the Company’s Series C redeemable convertible preferred stock at a future date was determined to be a freestanding financial instrument that should be accounted for as a liability. At initial recognition, the Company recorded the redeemable convertible preferred stock liability on the balance sheet at its estimated fair value. The liability was subject to remeasurement at each balance sheet date, with changes in fair value recognized as gain (loss) on remeasurement of redeemable convertible preferred stock liability on the consolidated statement of operations. Upon settlement of the redeemable convertible preferred stock liability in August 2019, the Company remeasured the liability and reclassified the final value associated with the redeemable convertible preferred stock liability to the carrying value of the Series C redeemable convertible preferred stock.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Valuation allowances are established when necessary to reduce deferred taxes to the amounts expected to be realized. The examination
Commitments and Contingencies	Commitments and Contingencies Liabilities for loss contingencies arising from claims, assessments, litigation, fines, and penalties and other sources are recorded when it is probable that a liability has been incurred and the amount can be reasonably estimated. Legal costs incurred in connection with loss contingencies are expensed as incurred.
Foreign Currencies	Foreign Currencies The Dollar Gains
Research and Development Expense	Research and Development Expense Research and development expenses consist primarily of direct and indirect costs incurred for the development of the Company’s product candidates. Direct expenses include (i) preclinical and clinical outside service costs associated with discovery, preclinical and clinical testing of the Company’s product candidates; (ii) professional services agreements with third-party contract organizations, investigative clinical trial sites and consultants that conduct research and development activities on the Company’s behalf; (iii) contract manufacturing costs to produce clinical trial materials; and (iv) laboratory supplies and materials. Indirect expenses include (A) compensation and personnel- related expenses (including stock-based compensation), (B) allocated expenses for facilities and depreciation; and (C) other indirect costs. Research and development costs are expensed as incurred. Payments made to third parties are under agreements that are generally cancelable by the Company. Advance payments for research and development activities are deferred as prepaid expenses. The prepaid amounts are expensed as the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the period over which services will be performed and the level of effort to be expended in each period. These estimates are based on the Company’s communications with the third-party service providers and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies significantly from the estimate, the Company will adjust
Stock-Based Compensation	Stock-Based Compensation The The Company grants certain employees performance-based stock options. For awards that include performance conditions, no compensation cost is recognized until the performance goals are probable of being met, at which time the cumulative compensation expense from the service inception date would be recognized. Stock-based compensation costs are based on the fair value of the underlying option calculated using the Black-Scholes option pricing model and recognized as expense on a straight-line basis (for all but performance based awards for which the accelerated method is used) over the requisite service period, which is the vesting period. Determining the appropriate fair value model and related assumptions requires judgment, including estimating the fair value of the underlying common stock, expected term, expected stock price volatility, risk-free interest rate and dividend yield. The Company accounts for forfeitures
Accounting for Non-Recurring Grant Income	Accounting for Non-Recurring Grant Income Non-recurring grant income is recognized when the research and development activities have been undertaken and the Company has completed its assessment of whether such activities meet the relevant qualifying criteria. Grants received from government and other agencies in advance of the specific research and development costs to which they relate are deferred and recognized in the consolidated statement of operations in the period they are earned and when the related research and development costs are incurred. Non-recurring grant income recognized in other income, net was $190,000 for the year ended December 31, 2019. No non-recurring grant income was recognized during 2020.
Net Loss Per Share Attributable to Common Stockholders	Net Loss Per Share Attributable Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. As the Company was in a loss position for all periods presented, basic net loss per share attributable to common stockholders is the same as diluted net loss per share attributable to common stockholders because the effects of potentially dilutive securities are antidilutive.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. The Company’s cash and Management
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 or the JOBS Act. Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (Topic 718) In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement (Topic 820) In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740). The guidance eliminates certain exceptions for recognizing deferred taxes for investments, performing intraperiod allocation and calculating income taxes in interim periods. This guidance also includes guidance to reduce complexity in certain areas, including recognizing deferred taxes for tax goodwill and allocating taxes to members of a consolidated group. ASU No. 2019-12 is effective for annual and interim periods in fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company early adopted this standard on January 1, 2020. The adoption of this standard did not have a material impact on the financial statements.
Recently Issued Accounting Pronouncements	Recently In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), , Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments - Credit Losses Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 825), Codification Improvements to Financial Instruments is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies. In August 2018, the FASB issued ASU No. 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract (Topic 350)

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Useful Lives of Property and Equipment	The useful Laboratory equipment 5 years Office and computer equipment 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following tables summarize the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): December 31, 2020 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 267,968 $ — $ — $ 267,968 Short-term investments: Commercial paper Level 2 59,930 — — 59,930 Corporate bonds Level 2 22,717 1 (7 ) 22,711 Total assets $ 350,615 $ 1 $ (7 ) $ 350,609 December 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 43,621 $ — $ — $ 43,621 Total assets $ 43,621 $ — $ — $ 43,621
Summary of Changes in the Carrying Value of the Liability	The changes in the carrying value of the liability were as follows (in thousands): Fair value as of December 31, 2018 $ 5,140 Change in fair value 5,670 Reclassification to redeemable convertible preferred stock upon settlement (10,810 ) Fair value as of December 31, 2019 $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Balance Sheet Components [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2020 2019 Prepaid insurance $ 1,236 $ 16 Prepaid research and development costs 1,039 1,086 Other prepaid expenses 527 294 Other receivables 3 79 Total prepaid expenses and other current assets $ 2,805 $ 1,475
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Leasehold improvements $ 3,061 $ 2,772 Laboratory equipment 555 456 Furniture and fixtures 263 187 Computer equipment and software 27 27 Total property and equipment, gross $ 3,906 $ 3,442 Less: accumulated depreciation (1,971 ) (1,304 ) Total property and equipment, net $ 1,935 $ 2,138
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2020 2019 Accrued research and development expenses $ 3,260 $ 459 Accrued compensation 2,543 926 Accrued professional services 524 733 Other accrued expenses 170 76 Total accrued liabilities $ 6,497 $ 2,194

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments	Future minimum lease payments under noncancelable operating leases as of December 31, 2020 were as follows (in thousands): 2021 $ 618 2022 4,189 2023 4,756 2024 5,035 2025 and thereafter 38,332 Total $ 52,930

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2020
Temporary Equity [Abstract]
Summary of Redeemable Convertible Preferred Stock	As of December 31, 2019, redeemable convertible preferred stock consisted of the following: Shares Authorized Shares Outstanding Net Carrying Value (in thousands) Liquidation Preference (in thousands) Series A 1,015,434 1,015,434 $ 1,000 $ 1,000 Series A-1 16,398,995 16,398,995 17,144 17,219 Series B 38,778,091 38,778,090 44,505 44,595 Series C 62,962,952 55,555,546 81,335 75,000 Total 119,155,472 111,748,065 $ 143,984 $ 137,814
Schedule of Common Stock Reserved for Future Issuance	The Company reserved the following shares of common stock for issuance as follows: December 31, 2020 2019 Redeemable convertible preferred stock outstanding on an as-converted basis — 12,684,214 Options issued and outstanding 3,909,873 2,007,222 Options available for future grant 2,707,947 454,110 Reserved for employee stock purchase plan 360,086 — Total common stock reserved 6,977,906 15,145,546

Equity Incentive Plan (Tables)

Equity Incentive Plan (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	Stock option activity under the 2011 Plan and the 2020 Plan was as follows: Shares Available for Grant Number of Shares Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances as of December 31, 2019 454,110 2,007,222 $ 4.91 8.26 $ 21,623 Additional shares authorized 4,166,016 — Stock options granted (1,914,819 ) 1,914,819 $ 16.88 Stock options exercised — (9,528 ) $ 2.33 Stock options cancelled 2,640 (2,640 ) $ 4.67 Balances as of December 31, 2020 2,707,947 3,909,873 $ 10.78 8.40 $ 56,128 Exercisable as of December 31, 2020 1,365,487 $ 4.96 6.92 $ 27,346
Summary of Stock-Based Compensation Expense	The total stock-based compensation expense recognized was as follows (in thousands): Year Ended December 31, 2020 2019 Research and development $ 2,274 $ 713 General and administrative 2,614 1,324 Total stock-based compensation expense $ 4,888 $ 2,037
Summary of Fair Value of Each Award Issued Estimated on the Date of Grant Using the Black-Scholes Option Pricing Model	The fair value of each award issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Year Ended December 31, 2020 2019 Expected term (in years) 5.00-6.08 6.02-6.08 Expected volatility 85%-93% 72%-77% Risk-free interest rate 0.33%-1.45% 1.48%-2.61% Dividend yield — —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Components of Loss Before Income Taxes	For financial reporting purposes, loss before provision for income taxes, includes the following components (in thousands): Year Ended December 31, 2020 2019 Domestic $ 63,406 $ 36,973 Foreign 6 206 Loss before income taxes $ 63,412 $ 37,179
Reconciliation of Income Tax Computed at Federal Statutory Rates to Provision for Income Taxes	Reconciliation of income tax computed at federal statutory rates to the reported provision for income taxes was as follows (in thousands): Year Ended December 31, 2020 2019 Tax provision at U.S. statutory rate $ (13,316 ) $ (7,808 ) State income taxes — 4 Stock-based compensation 426 112 R&D tax credits (1,031 ) (782 ) Change in valuation allowance 13,906 7,292 Redeemable convertible preferred stock liability — 1,191 Other 15 (5 ) Provision for income taxes $ — $ 4
Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to significant portions of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2020 2019 Deferred Tax Assets: Net operating loss carryforwards $ 36,922 $ 24,917 Research and development credits 6,246 4,405 Other intangibles 5 6 Accruals and reserves 583 254 Stock-based compensation 995 393 Tenant improvement allowances 257 325 Total gross deferred tax assets 45,008 30,300 Less: valuation allowance (44,745 ) (29,961 ) Total deferred tax assets, net $ 263 $ 339 Deferred Tax Liabilities: Fixed assets $ (263 ) $ (339 ) Total gross deferred tax liabilities (263 ) (339 ) Net deferred tax assets $ — $ —
Unrecognized Tax Benefits	The Company has the following activity relating to the gross amount of unrecognized tax benefits (in thousands): Year Ended December 31, 2020 2019 Beginning balance $ 1,116 $ 783 Additions based on tax positions related to current year 460 333 Ending balance $ 1,576 $ 1,116

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Shares Excluded From Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: Year Ended December 31, 2020 2019 Redeemable convertible preferred stock on an as-converted basis — 12,684,214 Stock options to purchase common stock 3,909,873 2,007,222 Shares subject to employee stock purchase plan 275 — Total 3,910,148 14,691,436

Organization - Additional Infor

Organization - Additional Information (Detail) $ / shares in Units, $ in Thousands	Aug. 04, 2020shares	Jul. 24, 2020USD ($)$ / sharesshares	Jul. 17, 2020	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)
Proceeds from issuance of common stock upon initial public offering, net of underwriting discounts and commissions		$ 262,400		$ 267,021
Payment of deferred offering costs				2,974	$ 1,605
Redeemable convertible preferred stock, shares issued upon conversion | shares		20,824,938
Accumulated deficit				(165,992)	$ (102,580)
Cash and cash equivalents and short-term investments				$ 351,200
Common Stock [Member]
Shares issued during the period | shares				16,889,403
Payment of deferred offering costs				$ 24,700
Description of reverse stock split			one-for-8.81 basis
Reverse stock split, conversion ratio			0.113507378
IPO [Member]
Shares issued during the period | shares		14,750,000
Shares issued price per share | $ / shares		$ 17
Payments for underwriting expense		$ 20,100
Payment of deferred offering costs		$ 4,600
Over-Allotment Option [Member]
Shares issued during the period | shares	2,139,403

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)Segment	Dec. 31, 2019USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment	1
Impairments of long-lived assets	$ 0
Deferred offering costs		$ 2,300,000
Interest or penalties charged in relation to unrecognized tax benefits	0
Foreign currency translation adjustment	$ 9,000	(14,000)
ASU 2018-07 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Change in accounting principle, ASU adopted	true
Change in accounting principle, ASU, adoption date	Jan. 1, 2020
Change in accounting principle, ASU, Immaterial effect	true
ASU 2018-13 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Change in accounting principle, ASU adopted	true
Change in accounting principle, ASU, adoption date	Jan. 1, 2020
Change in accounting principle, ASU, Immaterial effect	true
ASU 2019-12 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Change in accounting principle, ASU adopted	true
Change in accounting principle, ASU, adoption date	Jan. 1, 2020
Change in accounting principle, ASU, Immaterial effect	true
Other Income, Net [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Non-recurring grant income	$ 0	$ 190,000,000

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2020
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	5 years
Office and Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	3 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	Shorter of remaining lease term or estimated useful life

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 350,615	$ 43,621
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	(7)
Aggregate Fair Value	350,609	43,621
Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	267,968	43,621
Aggregate Fair Value	267,968	$ 43,621
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	59,930
Aggregate Fair Value	59,930
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	22,717
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	(7)
Aggregate Fair Value	$ 22,711

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment maturity period	2 years
Liability remeasured to its fair value			$ 5,140,000
Gain or loss on re-measurement of redeemable convertible preferred stock liability	$ 0	$ 5,670,000
Redeemable Convertible Preferred Stock Liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability remeasured to its fair value	$ 10,800,000
IPO Scenario [Member] | Redeemable Convertible Preferred Stock Liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Weighted average equity value percentage	50.00%
Staying Private Scenario [Member] | Redeemable Convertible Preferred Stock Liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Weighted average equity value percentage	50.00%

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Changes in the Carrying Value of the Liability (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Fair value as of December 31, 2018		$ 5,140,000
Change in fair value	$ 0	5,670,000
Reclassification to redeemable convertible preferred stock upon settlement		$ (10,810,000)

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Prepaid insurance	$ 1,236	$ 16
Prepaid research and development costs	1,039	1,086
Other prepaid expenses	527	294
Other receivables	3	79
Total prepaid expenses and other current assets	$ 2,805	$ 1,475

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 3,906	$ 3,442
Less: accumulated depreciation	(1,971)	(1,304)
Total property and equipment, net	1,935	2,138
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	3,061	2,772
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	555	456
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	263	187
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 27	$ 27

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Depreciation expense	$ 667,000	$ 493,000

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 3,260	$ 459
Accrued compensation	2,543	926
Accrued professional services	524	733
Other accrued expenses	170	76
Total accrued liabilities	$ 6,497	$ 2,194

Promissory Note - Additional In

Promissory Note - Additional Information (Detail) - USD ($) $ in Thousands	May 01, 2020	Apr. 21, 2020	Dec. 31, 2020
Short Term Debt [Line Items]
Proceeds from Paycheck Protection Program loan			$ 500
Repayments of Paycheck Protection Program loan			$ 500
U S Small Business Administration Loan [Member]
Short Term Debt [Line Items]
Proceeds from Paycheck Protection Program loan		$ 500
Repayments of Paycheck Protection Program loan	$ 500

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			12 Months Ended		49 Months Ended
	Dec. 31, 2020USD ($)Claim	Mar. 31, 2019USD ($)	Nov. 30, 2011USD ($)	Dec. 31, 2020USD ($)Claim	Dec. 31, 2019USD ($)	Dec. 31, 2020USD ($)Claim
Gain Contingencies [Line Items]
Operating leases, rent expense				$ 353,000	$ 353,000
Expensed to research and development				49,271,000	24,524,000
Income (loss)				(63,412,000)	(37,183,000)
Loss contingency accrual	$ 0			$ 0		$ 0
Indemnification Agreement [Member]
Gain Contingencies [Line Items]
Loss contingency pending claims | Claim	0			0		0
Stanford Agreement [Member] | Research and Development Arrangement [Member]
Gain Contingencies [Line Items]
Sponsor research agreements, commitment amount			$ 500,000
Expensed to research and development				$ 100,000
Sponsored Research Agreement [Member]
Gain Contingencies [Line Items]
Maximum threshold period allowed for filing market authorization		15 years
Proceeds from funding						$ 455,000
Income (loss)				$ 0
Sponsored Research Agreement [Member] | Other Income [Member]
Gain Contingencies [Line Items]
Proceeds from funding					$ 190,000
Sponsored Research Agreement [Member] | Research and Development Arrangement [Member]
Gain Contingencies [Line Items]
Maximum expected research funding		$ 651,000
South San Francisco, California [Member]
Gain Contingencies [Line Items]
Operating lease, term of contract	7 years			7 years		7 years
Operating lease, expiration month and year				Jun. 30, 2024
Operating lease, renewal term	5 years			5 years		5 years
Brisbane, California [Member]
Gain Contingencies [Line Items]
Operating lease, not yet commenced beginning month and year	2021-11
Operating lease, not yet commenced expiration month and year	2031-10
Operating lease, not yet commenced option to extend [true false]	true
Operating lease, not yet commenced extended additional term	10 years			10 years		10 years
Operating lease, base rent abatement period	3 months
Letter of credit	$ 1,000,000			$ 1,000,000		$ 1,000,000
Brisbane, California [Member] | Maximum [Member]
Gain Contingencies [Line Items]
Tenant improvement allowance receivable	$ 10,800,000			$ 10,800,000		$ 10,800,000

Commitments and Contingencies_2

Commitments and Contingencies - Future Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021	$ 618
2022	4,189
2023	4,756
2024	5,035
2025 and thereafter	38,332
Total	$ 52,930

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) - Summary of Redeemable Convertible Preferred Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Series A [Member]
Temporary Equity [Line Items]
Shares Authorized		1,015,434
Shares Outstanding		1,015,434
Net Carrying Value		$ 1,000
Liquidation Preference		$ 1,000
Series A-1 [Member]
Temporary Equity [Line Items]
Shares Authorized		16,398,995
Shares Outstanding		16,398,995
Net Carrying Value		$ 17,144
Liquidation Preference		$ 17,219
Series B [Member]
Temporary Equity [Line Items]
Shares Authorized		38,778,091
Shares Outstanding		38,778,090
Net Carrying Value		$ 44,505
Liquidation Preference		$ 44,595
Series C [Member]
Temporary Equity [Line Items]
Shares Authorized		62,962,952
Shares Outstanding		55,555,546
Net Carrying Value		$ 81,335
Liquidation Preference		$ 75,000
Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized	0	119,155,472
Shares Outstanding	0	111,748,065	89,525,848
Net Carrying Value		$ 143,984
Liquidation Preference	$ 0	$ 137,814

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 24, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Temporary Equity [Line Items]
Aggregate proceeds from redeemable convertible preferred stock			$ 102,000	$ 29,997
Net issuance cost			2,974	1,605
Recognized a beneficial conversion feature			705	$ 1,095
Conversion of redeemable preferred stock into common shares	20,824,938
Increase in the carrying value of redeemable convertible preferred stock	$ 6,200
Carrying value of the redeemable convertible preferred stock reclassified to equity	$ 241,400		$ 6,297
Common stock, voting rights			The holders of the Company’s common stock have one vote for each share of common stock.
Common stock, dividends declared			$ 0
Series D Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock issued		71,719,859	71,719,859
Redeemable convertible preferred stock issue price per share		$ 1.4222
Aggregate proceeds from redeemable convertible preferred stock		$ 96,800
Net issuance cost		5,200	$ 5,200
Recognized a beneficial conversion feature		$ 6,300
Series D Preferred Stock [Member] | Principal Owner [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock issued		9,773,587

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2020	Jul. 31, 2020	Jul. 24, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Redeemable convertible preferred stock outstanding on an as-converted basis			20,824,938
Options issued and outstanding	3,909,873			2,007,222
Options available for future grant	2,707,947			454,110
Common stock reserved	6,977,906			15,145,546
Reserved for Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	360,086	360,086
Redeemable Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Redeemable convertible preferred stock outstanding on an as-converted basis				12,684,214
Options Issued and Outstanding [Member]
Class Of Stock [Line Items]
Options issued and outstanding	3,909,873			2,007,222
Options Available for Future Grant [Member]
Class Of Stock [Line Items]
Options available for future grant	2,707,947			454,110

Equity Incentive Plan - Additio

Equity Incentive Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jul. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock for future issuance	6,977,906	15,145,546
Percentage of number of shares of capital stock outstanding on last day of preceding year	4.00%
Stock option grants description	Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For the Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.
Option expiry period	10 years
Vesting Period	4 years
Options available for future grant	2,707,947	454,110
Intrinsic value of options exercised	$ 181,000	$ 8,000
Weighted- average exercise price per share, Stock options granted	$ 16.88
Unrecognized stock-based compensation cost	$ 25,800,000
Unrecognized stock-based compensation cost expected period for recognition	3 years 21 days
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted- average exercise price per share, Stock options granted	$ 12.33	$ 5.72
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock for future issuance	360,086		360,086
Purchase of Common stock at end of each offering period at price equal to percentage of fair market value of shares	85.00%
Share purchases funded through payroll deductions minimum percentage of employee’s eligible compensation	1.00%
Share purchases funded through payroll deductions maximum percentage of employee’s eligible compensation	15.00%
Percentage of shares of common stock outstanding on last day of immediately preceding fiscal year	1.00%
Employee Stock Purchase Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock for future issuance	360,086
2020 Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock for future issuance			3,600,868
Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of Common stock at end of each offering period at price equal to percentage of fair market value of shares	100.00%

Equity Incentive Plan - Summary

Equity Incentive Plan - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares available for grant, Beginning balance	454,110
Shares available for grant, Additional shares authorized	4,166,016
Shares available for grant, Stock options granted	(1,914,819)
Shares available for grant, Stock options exercised	(9,528)
Shares available for grant, Stock options cancelled	2,640
Shares available for grant, Ending balance	2,707,947	454,110
Shares available for grant, Exercisable	1,365,487
Number of shares, Beginning balance	2,007,222
Number of shares, Stock options granted	1,914,819
Number of shares, Stock options cancelled	(2,640)
Number of shares, Ending balance	3,909,873	2,007,222
Weighted- average exercise price per share, Beginning balance	$ 4.91
Weighted- average exercise price per share, Stock options granted	16.88
Weighted- average exercise price per share, Stock options exercised	2.33
Weighted- average exercise price per share, Stock options cancelled	4.67
Weighted- average exercise price per share, Ending balance	10.78	$ 4.91
Weighted- average exercise price per share, Exercisable	$ 4.96
Weighted- average remaining contractual term	8 years 4 months 24 days	8 years 3 months 3 days
Weighted- average remaining contractual term, Exercisable	6 years 11 months 1 day
Aggregate intrinsic value	$ 56,128	$ 21,623
Aggregate intrinsic value, Exercisable	$ 27,346

Equity Incentive Plan - Summa_2

Equity Incentive Plan - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 4,888	$ 2,037
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	2,274	713
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,614	$ 1,324

Equity Incentive Plan - Summa_3

Equity Incentive Plan - Summary of Fair Value of Each Award Issued Estimated on the Date of Grant Using the Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	85.00%	72.00%
Expected volatility, Maximum	93.00%	77.00%
Risk-free interest rate, Minimum	0.33%	1.48%
Risk-free interest rate, Maximum	1.45%	2.61%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years	6 years 7 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days

Income Taxes - Components of Lo

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Domestic	$ 63,406	$ 36,973
Foreign	6	206
Loss before income taxes	$ 63,412	$ 37,179

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Income Tax Computed at Federal Statutory Rates to Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Tax provision at U.S. statutory rate	$ (13,316)	$ (7,808)
State income taxes		4
Stock-based compensation	426	112
R&D tax credits	(1,031)	(782)
Change in valuation allowance	13,906	7,292
Redeemable convertible preferred stock liability		1,191
Other	$ 15	(5)
Provision for income taxes		$ 4

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 36,922	$ 24,917
Research and development credits	6,246	4,405
Other intangibles	5	6
Accruals and reserves	583	254
Stock-based compensation	995	393
Tenant improvement allowances	257	325
Total gross deferred tax assets	45,008	30,300
Less: valuation allowance	(44,745)	(29,961)
Total deferred tax assets, net	263	339
Fixed assets	(263)	(339)
Total gross deferred tax liabilities	$ (263)	$ (339)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax [Line Items]
Federal net operating loss carryforwards	$ 146,700,000
State net operating loss carryforwards	$ 66,700,000
Percentage of taxable income	80.00%
State carryforward losses expiration beginning year	2031
Increase in valuation allowance	$ 14,800,000	$ 10,400,000
Uncertain tax position	0
Unrecognized tax benefits, interest or penalty accruals	$ 0
Earliest Tax Year [Member]
Income Tax [Line Items]
Open tax year examination	2011
Latest Tax Year [Member]
Income Tax [Line Items]
Open tax year examination	2020
Federal [Member]
Income Tax [Line Items]
Federal NOLs do not expire	$ 103,600,000
Tax credit carryforwards	$ 4,300,000
Federal carryforward losses expiration beginning year	2031
State [Member]
Income Tax [Line Items]
Tax credit carryforwards	$ 3,500,000

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Beginning balance	$ 1,116	$ 783
Additions based on tax positions related to current year	460	333
Ending balance	$ 1,576	$ 1,116

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Summary of Outstanding Potentially Dilutive Shares Excluded From Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	3,910,148	14,691,436
Employees [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	275
Redeemable Convertible Preferred Stock on an as-converted Basis [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share		12,684,214
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	3,909,873	2,007,222