Alector (ALEC) 10-K 25 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 15, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	82-2933343
Entity Public Float			$ 1,375.6
Entity Address, Address Line One	131 Oyster Point Blvd
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	415
Local Phone Number	231-5660
Document Transition Report	false
Document Annual Report	true
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		79,584,442
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Incorporation, State or Country Code	DE
Documents Incorporated by Reference	Portions of the registrant’s Definitive Proxy Statement relating to the registrant’s Annual Meeting of Shareholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 49,969	$ 89,641
Marketable securities	363,339	263,432
Prepaid expenses and other current assets	8,203	4,364
Total current assets	421,511	357,437
Property and equipment, net	30,181	33,852
Operating lease right-of-use assets	32,470	28,476
Restricted cash	1,472	1,472
Other assets	2,617	676
Total assets	488,251	421,913
Current liabilities:
Accounts payable	3,004	278
Accrued clinical supply costs	11,148	6,706
Accrued liabilities	22,538	18,256
Deferred revenue, current portion	23,886	30,165
Operating lease liabilities, current portion	7,512	6,565
Total current liabilities	68,088	61,970
Deferred revenue, long-term portion	108,417	123,236
Operating lease liabilities, long-term portion	43,744	41,471
Other long-term liabilities	472	493
Total liabilities	220,721	227,170
Commitments and contingencies (Note 5)
Stockholders' equity:
Common stock, $0.0001 par value; 200,000,000 shares authorized; 79,316,261 and 69,052,873 shares issued and outstanding as of December 31, 2020 and 2019, respectively	8	7
Additional paid-in capital	676,956	414,414
Accumulated other comprehensive income	614	142
Accumulated deficit	(410,048)	(219,820)
Total stockholders' equity	267,530	194,743
Total liabilities and stockholders' equity	$ 488,251	$ 421,913

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	79,316,261	69,052,873
Common stock, shares outstanding	79,316,261	69,052,873

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Total revenue	$ 21,098	$ 21,219	$ 27,508
Type of Revenue [Extensible List]		alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Grant revenue			$ 169
Total revenue	21,098	$ 21,219	27,677
Operating expenses:
Research and development	156,869	100,528	73,031
General and administrative	59,403	35,095	11,934
Total operating expenses	216,272	135,623	84,965
Loss from operations	(195,174)	(114,404)	(57,288)
Other income, net	4,946	9,019	5,040
Net loss	(190,228)	(105,385)	(52,248)
Unrealized gain (loss) on marketable securities	472	184	(42)
Comprehensive loss	$ (189,756)	$ (105,201)	$ (52,290)
Net loss per share, basic and diluted	$ (2.45)	$ (1.71)	$ (4.62)
Shares used in computing net loss per share, basic and diluted	77,758,806	61,734,492	11,302,788

Consolidated Statement of Conve

Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2017	$ (52,033)		$ 1	$ 10,153		$ (62,187)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2017		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017			13,776,153
Issuance of Series E convertible preferred stock, net of issuance costs		$ 133,035
Issuance of Series E convertible preferred stock, net of costs (in shares)		9,373,633
Forfeiture/Cancellation of restricted common stock			(11,324)
Stock-based compensation	6,925			6,925
Unrealized gain (loss) on marketable securities	(42)				$ (42)
Net loss and comprehensive loss	(52,248)					(52,248)
Ending Balance at Dec. 31, 2018	(97,398)		$ 1	17,078	(42)	(114,435)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2018		45,374,836
Convertible preferred stock, ending balance at Dec. 31, 2018		$ 210,520
Ending balance (in shares) at Dec. 31, 2018			13,764,829
Conversion of convertible preferred stock into common stock	210,521	$ (210,520)	$ 5	210,516
Conversion of convertible preferred stock into common stock (in shares)		(45,374,836)	45,374,836
Issuance of common stock upon initial public offering, net of issuance costs	168,223		$ 1	168,222
Issuance of common stock upon initial public offering, net of issuance costs (in shares)			9,739,541
Exercise of stock options	1,519			1,519
Exercise of stock options (in shares)			180,287
Purchase of common stock under employee stock purchase plan	798			798
Purchase of common stock under employee stock purchase plan (in shares)			50,353
Forfeiture/Cancellation of restricted common stock			(56,973)
Stock-based compensation	16,281			16,281
Unrealized gain (loss) on marketable securities	184				184
Net loss and comprehensive loss	(105,385)					(105,385)
Ending Balance at Dec. 31, 2019	$ 194,743		$ 7	414,414	142	(219,820)
Ending balance (in shares) at Dec. 31, 2019	69,052,873		69,052,873
Issuance of common stock upon initial public offering, net of issuance costs	$ 224,511		$ 1	224,510
Issuance of common stock upon initial public offering, net of issuance costs (in shares)			9,602,500
Exercise of stock options	6,331			6,331
Exercise of stock options (in shares)			655,772
Purchase of common stock under employee stock purchase plan	1,179			1,179
Purchase of common stock under employee stock purchase plan (in shares)			86,870
Forfeiture/Cancellation of restricted common stock			(81,754)
Stock-based compensation	30,522			30,522
Unrealized gain (loss) on marketable securities	472				472
Net loss and comprehensive loss	(190,228)					(190,228)
Ending Balance at Dec. 31, 2020	$ 267,530		$ 8	$ 676,956	$ 614	$ (410,048)
Ending balance (in shares) at Dec. 31, 2020	79,316,261		79,316,261

Consolidated Statement of Con_2

Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Issuance cost of common stock	$ 1,148	$ 3,874
Series E Preferred Stock [Member]
Issuance cost of convertible preferred Stock			$ 214

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (190,228)	$ (105,385)	$ (52,248)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,870	3,775	1,019
Stock-based compensation	30,522	16,281	6,925
Amortization of premiums and accretion of discounts on marketable securities	584	(4,701)	(2,745)
Amortization of right-of-use assets	1,395	1,799
Impairment loss on right-of-use assets	238	1,158
Loss from disposal of property and equipment, net	107	39	151
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,839)	(1,596)	(2,245)
Other assets	(2,498)	(42)	106
Accounts payable	2,409	152	(809)
Accrued liabilities and accrued clinical supply costs	12,232	9,210	4,743
Deferred revenue	(21,098)	(21,219)	172,492
Lease liabilities	(2,407)	868
Other long-term liabilities	(21)	353	75
Net cash provided by (used in) operating activities	(166,734)	(99,308)	127,464
Cash flows from investing activities:
Purchase of property and equipment	(5,032)	(15,265)	(1,884)
Purchase of marketable securities	(506,809)	(529,609)	(472,235)
Maturities of marketable securities	406,790	496,000	250,000
Net cash used in investing activities	(105,051)	(48,874)	(224,119)
Cash flows from financing activities:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs		170,128	(1,904)
Proceeds from issuance of common stock upon follow-on public offering, net of issuance cost	224,603	(92)
Proceeds from issuance of convertible preferred stock, net of issuance costs			133,050
Proceeds from the exercise of options to purchase common stock	6,331	1,519
Proceeds from issuance of stock from employee stock purchase plan	1,179	798
Net cash provided by financing activities	232,113	172,353	131,146
Net increase (decrease) in cash, cash equivalents, and restricted cash	(39,672)	24,171	34,491
Cash, cash equivalents, and restricted cash at beginning of period	91,113	66,942	32,451
Cash, cash equivalents, and restricted cash at end of period	51,441	91,113	66,942
Non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	$ 428	3,471	293
Deferred offering costs in accrued liabilities		465	792
Tenant improvements paid by landlord		$ 8,286	$ 7,449

The Company and Liquidity

The Company and Liquidity	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company and Liquidity	1. Alector, Inc. (Alector or the Company) is a Delaware corporation headquartered in South San Francisco, California. Alector is a clinical stage biopharmaceutical company pioneering immuno-neurology, a novel therapeutic approach for the treatment of neurodegeneration. Initial Public and Follow-on Offerings On February 7, 2019, the Company completed an initial public offering (IPO) through issuing and selling 9,739,541 shares of common stock at a public offering price of $19.00 per share, including 489,541 shares sold pursuant to the underwriters’ partial exercise of their option to purchase additional shares, resulting in aggregate net proceeds of $168.2 million, after deducting underwriting discounts and commissions and offering costs. Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 45,374,836 shares of common stock. Subsequent to the closing of the IPO, there were no shares of convertible preferred stock outstanding. On January 30, 2020, the Company completed a follow-on offering through issuing and selling 9,602,500 shares of common stock at a public offering price of $25.00 per share, including 1,252,500 shares sold pursuant to the underwriters’ full exercise of their option to purchase additional shares, resulting in aggregate net proceeds of $224.5 million, after deducting underwriting discounts and commissions and estimated offering costs.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Basis of Presentation The consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States (GAAP) as defined by the Financial Accounting Standards Board (FASB). The consolidated financial statements include the accounts of Alector, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expense during the reporting period. The Company evaluates its estimates, including those related to revenue recognition, manufacturing accruals, clinical accruals, fair value of assets and liabilities, income taxes uncertainties, stock-based compensation, and related 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 materially differ from those estimates . Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, and short-term marketable securities. Cash and cash equivalents are deposited in checking and sweep accounts at a financial institution. Such deposits may, at times, exceed federally insured limits. Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash and cash equivalents. Cash equivalents, which consist of amounts invested in money market funds, are stated at fair value. Restricted cash as of December 31, 2020 relates to a letter of credit established for a lease entered into in June 2018. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: Year Ended December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 49,969 $ 89,641 $ 65,470 Restricted cash 1,472 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 51,441 $ 91,113 $ 66,942 Marketable Securities All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. The Company considers its available-for-sale portfolio as available for use in current operations. Accordingly, the Company may classify certain investments as short-term marketable securities, even though the stated maturity date may be one year or more beyond the current balance sheet date. For available-for-sale debt securities, unrealized gains, net of any related tax effects, are excluded from earnings and are included in other comprehensive income and reported as a separate component of stockholders’ equity (deficit) until realized. The Company assesses available-for-sale debt securities on a quarterly basis to see if any unrealized loss is due to credit-related factors. Factors considered in determining whether an impairment is credit-related include the extent to which the investment’s fair value is less than its cost basis, declines in published credit ratings, changes in interest rates, and any other adverse factors related to the security. If it is determined that a credit-related impairment exists, the Company will measure the credit loss based on a discounted cash flows model. Credit-related impairments on available-for-sale debt securities are recognized as an allowance for credit losses with a corresponding adjustment to other income, net in the Company’s consolidated statement of operations. The unrealized loss position that is not credit-related is recorded, net of any related tax effects, in other comprehensive income until realized. There were no credit-related losses recognized for the periods presented. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. In accordance with our investment policy, management invests in money market funds, U.S. treasury securities, corporate bonds, commercial paper, and overnight repurchase agreements collateralized by U.S. treasury securities. The Company has not experienced any losses on its deposits of cash, cash equivalents, and marketable securities. Fair Value of Financial Instruments The Company’s believes the carrying value of financial instruments, including cash and cash equivalents, receivables, accounts payable, and accrued liabilities, approximate fair value due to their short-term nature. 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 the fair value of its financial instruments 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 are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the lesser of their useful lives or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statements of operations in the period realized. Maintenance and repairs are charged to the consolidated statements of operations as incurred. Leases The Company adopted Accounting Standards Update No. 2016-02, Leases The Company determines whether an arrangement is or contains a lease at the inception of the lease. Leases are recognized on the balance sheet as right-of-use assets and lease liabilities. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and any prepaid or accrued rent. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company excludes balance sheet recognition of operating leases having a term of 12 months or less (short-term leases) and does not separate lease components and non-lease components for its long-term leases. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If the total of the undiscounted future cash flows is less than the carrying amount of the assets, an impairment loss is recognized for the amount by which the carrying amount of the assets exceeds is fair value. For the year ended December 31, 2020, the Company recognized a $0.2 million impairment loss on its right-of-use asset related to decision to no longer utilize certain property through the end of the lease term. For the year ended December 31, 2019, the Company recognized a $1.2 million impairment loss on its right of use assets related to the portion of its headquarters being subleased. Revenue Recognition The Company recognizes revenue when control of promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under arrangements, the Company performs the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies the performance obligation. Collaboration Revenue The Company signed an agreement in October 2017 with AbbVie Biotechnology, Ltd. (AbbVie) to co-develop antibodies to two program targets in preclinical development (AbbVie Agreement). Under the terms of the AbbVie Agreement, AbbVie made $205.0 million in upfront payments, of which $5.0 million and $200.0 million was received by the Company in October 2017 and January 2018, respectively. The Company will perform research and development services for the antibodies to the two programs through the end of Phase 2 clinical trials which the Company expects to conduct through 2023. AbbVie will then have the exclusive right to exercise an option to enter into a license and collaboration agreement with the Company for one or both of the programs for $250.0 million each. If AbbVie exercises its option for the programs, AbbVie will take over the development of the product candidates for such program and costs will be split between the parties. The Company will also share in profits and losses upon commercialization of any products from such program. However, following AbbVie’s exercise of its option for a program, the Company may opt out of sharing in development costs and profits or losses for that program and instead receive tiered royalties. Additionally, under the terms of the AbbVie Agreement, the Company will be eligible to earn up to an additional $242.8 million in milestone payments per program related to the initiation of certain clinical studies and regulatory approval for up to three indications per program. The Company assessed its collaboration agreement with AbbVie in the context of the delivery of the research and development services. The Company has determined that there are two research and development performance obligations as part of the agreement with AbbVie, one research and development performance obligation for each of the two research and development programs. The non-refundable upfront cash payments of $5.0 million and $200.0 million received in October 2017 and January 2018, respectively, were included in the transaction price. None of the remaining development and regulatory milestone and program opt-in payment amounts have been included in the transaction price, as all these amounts were fully constrained as of December 31, 2020 and 2019. As part of the Company’s evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestone amounts is outside the control of the Company and contingent upon success in future clinical trials. Any consideration related to royalties on net product sales will be recognized when the related sales occur and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company recognizes collaboration revenue by measuring the progress toward complete satisfaction of the performance obligation using an input measure. In order to recognize revenue over the research and development period, the Company measures actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligation. Revenues are recognized as the program costs are incurred. The Company will re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Clinical trials are expensive and can take many years to complete, and the outcome is inherently uncertain. Changes in the Company’s forecasted costs are likely to occur over time based upon changes in clinical trial procedures set forth in protocols, changes in estimates of manufacturing costs, or feedback from regulators on the design or operation of clinical trials. The Company has had changes to the overall expected costs to satisfy the performance obligations from period to period. For the year ended December 31, 2020, the Company had a 7% increase in the forecast of total expected costs due to changes to the clinical trial plans. For the year ended December 31, 2020, the increase in the overall expected costs to satisfy the performance obligation resulted in an approximately $6.7 million reduction in revenue compared to if the expected costs had remained the same, as a result of the cumulative catch up for the change in estimate. Collaboration revenue under the Company’s collaboration agreement with AbbVie during 2020, 2019, and 2018 was $21.1 million, $21.2 million, and $27.5 million, respectively. Collaboration revenue recognized during the year ended December 31, 2020, was included in deferred revenue at the beginning of the period. The balance of deferred revenue was $132.3 million as of December 31, 2020. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of Phase 2 clinical trials. The Company entered into an agreement in March 2020 with Innovent Biologics (Innovent) to license, develop, and commercialize AL008 in China (Innovent Agreement). AL008 is the Company’s novel antibody targeting the CD47-SIRP-alpha pathway, a potent survival pathway co-opted by tumors to evade the innate immune system. Under the terms of the Innovent Agreement, Innovent may pay the Company up to $11.5 million in development milestones, $112.5 million in sales milestones, and future royalties for any sales. The Company retains the rights to develop and commercialize AL008 outside of China. The Company has determined there is one performance obligation for the delivery of the license and will recognize revenue when it is probable that there will not be significant reversal of cumulative revenue. Development and sales milestones under the Innovent Agreement have not been included in the transaction price, as all these amounts were fully constrained as of December 31, 2020. As of December 31, 2020, no revenue has been recognized or payments received under the Innovent Agreement. Research and Development Costs Research and development costs are expensed as incurred and consist primarily of new product development. Research and development costs include salaries and benefits, consultants’ fees, process development costs, stock-based compensation, and laboratory supplies, as well as fees paid to third parties that conduct certain research and development activities on the Company’s behalf. In addition, research and development costs include the reimbursable costs incurred for the grant agreements, which includes payroll costs for time incurred on projects, laboratory supplies, and third-party research and development activities. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers. The Company records accrued expenses for estimated preclinical study and clinical trial expenses. Estimates are based on the services performed pursuant to contracts with research institutions, contract research organizations in connection with clinical studies, investigative sites in connection with clinical studies, vendors in connection with preclinical development activities, and contract manufacturing organizations in connection with the production of materials for clinical trials. Further, the Company accrues expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. The Company monitors patient enrollment levels and related activity to the extent reasonably possible and make judgments and estimates in determining the accrued balance in each reporting period. If the Company underestimates or overestimates the level of services performed or the costs of these services, actual expenses could differ from estimates. To date, the Company has not experienced significant changes in its estimates of preclinical studies and clinical trial accruals. Stock-based Compensation Stock-based compensation for employee awards is measured on the grant date based on the fair value of the award and recognized on a straight-line basis over the requisite service period. The fair value of options to purchase common stock are measured using the Black-Scholes option-pricing model. The Company accounts for forfeitures as they occur. Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are the result of transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive loss was net unrealized gain (loss) on marketable securities. Income Taxes The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have The deferred tax assets are recognized to the extent the Company believes that these assets are more likely than not to be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior periods, the net deferred tax assets have been fully offset by a valuation allowance. The Company records uncertain tax positions using a two-step process. First, the Company determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position. Second, for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits on the interest expense line and other expense line, respectively, in the accompanying statements of operations. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Employee 401(k) Plan The Company has a qualified contributory savings plan under Section 401(k) of the Internal Revenue Code (the “Code”) covering substantially all U.S. employees of Alector. The 401(k) plan is designed to provide tax-deferred retirement benefits in accordance with the provisions of Section 401(k) of the Code. Eligible employees may defer up to 100% of their eligible compensation up to the annual maximum as determined by the Internal Revenue Service. The Company’s contributions to the plan are discretionary. For the year ended December 31, 2020, the Company made matching contributions of $0.7 million. The Company did not make any matching contributions for the years ended December 31, 2019 or 2018. Segments The Company operates in one segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. Recent Accounting Pronouncements Effective January 1, 2020, the Company adopted Accounting Standards Update (ASU) No. 2016-13, Financial Instruments—Credit Losses (Topic 326) In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740) Simplifying the Accounting for Income Taxes.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables summarize the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2020 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 37,951 $ — $ — $ 37,951 U.S. government treasury securities Level 1 357,725 620 (6 ) 358,339 Corporate bonds Level 2 5,000 — — 5,000 Total cash equivalents and marketable securities $ 400,676 $ 620 $ (6 ) $ 401,290 December 31, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 61,104 $ — $ — $ 61,104 U.S. government treasury securities Level 1 227,446 149 (8 ) 227,587 Commercial paper Level 2 43,735 — — 43,735 Corporate bonds Level 2 10,103 3 (2 ) 10,104 Total cash equivalents and marketable securities $ 342,388 $ 152 $ (10 ) $ 342,530 The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models for which all significant inputs are observable. There were no transfers between levels of the fair value hierarchy during the year ended December 31, 2020. The Company classifies marketable securities available to fund current operations as current assets. As of December 31, 2020, the remaining contractual maturities of $336.9 million investments were less than one year and $64.4 million investments were after one year through two years. The Company does not intend to sell the investments that are in an unrealized loss position before recovery of their amortized cost basis, which may be maturity.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Property and Equipment, Net Property and equipment, net consists of the following: December 31, 2020 2019 (In thousands) Leasehold improvements $ 25,542 $ 25,222 Lab equipment 11,610 10,458 Furniture and fixtures 2,222 2,031 Computer equipment 1,764 1,554 Property and equipment, gross 41,138 39,265 Less accumulated depreciation and amortization (10,957 ) (5,413 ) Total property and equipment, net $ 30,181 $ 33,852 Accrued Liabilities Accrued liabilities consist of the following: December 31, 2020 2019 (In thousands) Accrued research and development costs $ 13,375 $ 6,400 Accrued employee compensation 7,288 5,730 Accrued professional services 1,094 2,040 Accrued property and equipment 428 3,471 Other 353 615 Total accrued liabilities $ 22,538 $ 18,256

Commitment and Contingencies

Commitment and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. On June 18, 2019, the Company initiated a confidential arbitration proceeding against Dr. Asa Abeliovich, the Company’s former consulting co-founder, related to alleged breaches of his consulting agreement and the improper use of Alector’s confidential information that he learned during the course of rendering services to the Company as consulting Chief Scientific Officer/Chief Innovation Officer. An independent arbitrator issued a confidential decision in favor of Alector, finding Dr. Abeliovich liable for breach of his confidentiality agreement and for spoliation based on his destruction of documents relevant to the proceeding. The arbitrator awarded damages for breach of the agreement and sanctions for the spoliation, as well as violation of orders during the proceeding, and the amount of such monetary damages will be determined in a pending further proceeding.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	6. In June 2018, the Company signed a lease agreement to lease approximately 105,000 square feet in office and laboratory space in South San Francisco which serves as the Company’s headquarters (the “Headquarters”). The lease expires in 2029 with an option to renew for a period of ten years. The landlord paid for $15.7 million of tenant improvements. In connection with the lease, the Company entered into a letter of credit arrangement in the amount of $1.5 million as collateral for the lease, which is classified as restricted cash on the consolidated balance sheets. In October 2020, the Company signed a lease agreement to lease approximately 18,700 square feet of office and laboratory space in Newark, California. The lease term ends on February 6, 2028 with an option to extend for an additional five years. The landlord is obligated to pay for up to $0.4 million of tenant improvements. The measurement of the lease liabilities for the leases excludes the options to extend the term of the lease as such extensions are not reasonably certain to occur. Variable lease costs for all of the Company’s leases consist of operating expenses for the spaces. In May 2019, the Company entered into an agreement to sublease approximately 25,000 square feet of the Headquarters (the “Sublease”), which was amended to approximately 23,600 square feet in December 2020. The Sublease expires in November 2021. The sublessor has the option to extend the Sublease for one additional year subject to the consent of the Company. The sublessee is required to pay its proportionate share of operating expenses for the space. In June 2019, in connection with the Sublease, the Company evaluated the related right-of-use asset for impairment. The lease costs plus amortization expense of the leasehold improvements in the subleased space exceeded the sublease income over the remaining sublease term. As such, the Company recorded an impairment charge of $1.2 million in general and administrative expenses to write-down the right-of-use asset such that no loss is expected to be recognized over the sublease period. The components of lease expense were as follows: December 31, 2020 2019 (In thousands) Operating lease cost $ 5,589 $ 5,875 Variable lease cost 2,255 1,328 Short-term lease cost — 68 Sublease income and reimbursement of variable lease cost (2,288 ) (1,263 ) Total $ 5,556 $ 6,008 Rent expense under legacy guidance for the year ended December 31, 2018 was $1.1 million. As of December 31, 2020, the weighted-average remaining lease term for operating leases was 8.2 years and the weighted-average discount rate was 8.5%. Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2020 and 2019 was $6.9 million and $3.2 million, respectively, was included in net cash used in operating activities in our consolidated statements of cash flows. The following are the lease payments owed under the Company’s operating leases as of December 31, 2020: (In thousands) 2021 $ 7,872 2022 8,171 2023 8,436 2024 8,732 2025 9,037 Thereafter 30,130 Total undiscounted lease payments $ 72,378 Less: Present value adjustment (21,122 ) Total lease liability $ 51,256

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7 . The Company recognized stock-based compensation as follows: Year Ended December 31, 2020 2019 2018 (In thousands) Research and development $ 14,980 $ 8,031 $ 4,195 General and administrative 15,542 8,250 2,730 Total stock-based compensation $ 30,522 $ 16,281 $ 6,925 Determination of Fair Value The estimated grant-date fair value of all the Company’s options to purchase common stock was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2020 2019 2018 Expected term (in years) 5.3-6.1 5.3 – 6.1 5.1 – 6.1 Expected volatility 74%-78% 76% – 82% 73% – 81% Risk-free interest rate 0.3% - 1.8% 1.4% – 2.6% 2.5% – 3.1% Dividend yield 0% 0% 0% The fair value of each stock option was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. Expected Term— The expected term represents the period that stock-based awards are expected to be outstanding. The expected term was derived by using the simplified method which uses the midpoint between the average vesting term and the contractual expiration period of the stock-based award. Expected Volatility— The Company has limited information on the volatility of stock options as the shares were not actively traded on any public markets prior to February 7, 2019. The expected volatility was derived from the historical stock volatilities of comparable peer public companies within its industry. These companies are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards. In 2020, the Company began giving weight to in its own historical volatility in the determination of expected volatility. Risk-Free Interest Rate— The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the expected term. Expected Dividend Rate— The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its common stock underlying its stock options in the foreseeable future. Restricted Common Stock The restricted common stock generally vests over a four-year period with straight-line vesting with a 25% one-year cliff. Activity for the restricted common stock is shown below: Number of Shares Weighted Average Grant Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 1,917,848 $ 6.95 Vested (866,037 ) 6.95 Forfeited (56,973 ) 6.95 Unvested restricted common stock as of December 31, 2019 994,838 6.95 Vested (726,659 ) 6.95 Forfeited (81,754 ) 6.95 Unvested restricted common stock as of December 31, 2020 186,425 6.95 As of December 31, 2020, total unrecognized stock-based compensation related to unvested restricted common stock issued to employees was $0.9 million, which the Company expects to recognize over a remaining weighted-average period of 0.8 years. 2019 Equity Incentive Plan On February 6, 2019, the Company adopted the 2019 Equity Incentive Plan (2019 Plan) under which the Board may issue incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units, and performance shares to the Company’s employees, directors, and consultants. The Company’s 2017 Stock Option and Grant Plan (2017 Plan) was terminated; however, shares subject to awards granted under it will continue to be governed by the 2017 Plan. Shares reserved for issuance but not issued pursuant to, or not subject to, awards granted under the 2017 Plan were added to the available shares in the 2019 Plan. Shares subject to awards granted under the 2017 Plan that are repurchased by, or forfeited to, the Company will also be reserved for issuance under the 2019 Plan. The board of directors, or a committee appointed by the board of directors, has the authority to determine to whom options or shares will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the exercise price of each share shall be at least 110% of the fair market value and the term of the award shall not exceed five years. All other options granted under the 2019 Plan must have an exercise price at least equal to the fair market value on the date of grant and have a term not to exceed ten years. The shares generally vest over a four-year period with straight-line vesting with a 25% one-year cliff. As of December 31, 2020, the Company had reserved 15,628,217 shares of common stock for issuance under the 2019 Plan, of which 2,973,219 shares were available for issuance. Activity for the options to purchase common stock shown below: Number of Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In years) (In thousands) Outstanding as of December 31, 2018 5,063,688 $ 8.94 Granted 3,738,071 17.74 Exercised (180,287 ) 8.43 Forfeited (178,648 ) 11.52 Outstanding as of December 31, 2019 8,442,824 12.79 Granted 5,597,148 15.35 Exercised (655,772 ) 9.65 Forfeited (729,202 ) 14.96 Outstanding as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633 Exercisable as of December 31, 2020 3,588,568 $ 12.74 8.1 $ 13,393 Vested and expected to vest as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633 The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. The aggregate intrinsic value of options exercised was $11.1 million and $1.8 million for the years ended December 31, 2020 and 2019. There were no options exercised in 2018. The weighted-average grant-date fair value per share of options granted was $10.14, $12.06, and $6.27, for the years ended December 31, 2020, 2019, and 2018, respectively. As of December 31, 2020, total unrecognized stock-based compensation related to unvested stock options was $82.9 million, which the Company expects to recognize over a remaining weighted-average period of 2.6 years. 2019 Employee Stock Purchase Plan On February 6, 2019, the Company adopted the 2019 Employee Stock Option Plan (2019 ESPP). The 2019 ESPP will enable eligible employees of the Company to purchase shares of common stock at a discount. Each offering period will be approximately six months long beginning on the first trading day on or after December 1 and June 1 each year. ESPP participants purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the first trading day of the offering period or (2) the fair market value of the common stock on the purchase date. As of December 31, 2020, there was $0.4 million in unrecognized compensation expense related to the 2019 ESPP to be recognized over five months.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	8 . The Company incurred net operating losses for the years ended December 31, 2020, 2019, and 2018. The Company has not reflected a benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its net deferred tax assets due to the uncertainty surrounding realization of such assets. A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2020, 2019, and 2018 (in thousands): Year Ended December 31, 2020 2019 2018 Tax benefit at federal statutory rate $ (39,948 ) $ (22,081 ) $ (10,972 ) State income taxes (16,570 ) (4,694 ) — Tax credits (8,010 ) (6,530 ) (2,387 ) Uncertain tax positions 2,003 2,044 708 Stock-based compensation 353 1,493 1,251 Nondeductible expense 51 124 58 Tax benefit recognized on utilization of attributes previous reserved — (13,582 ) — Deferred adjustment from amended tax return — (1,073 ) — Change in valuation allowance 62,280 44,748 11,346 Others (159 ) (449 ) (4 ) Income tax provision $ — $ — $ — The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets consist of (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss $ 54,382 $ 2,223 Accrued bonus 1,201 370 Tax credits 13,000 7,319 Stock-based compensation 6,493 2,015 Deferred revenue 32,129 37,621 Lease liability 12,457 11,781 Others 463 209 Gross deferred tax assets 120,125 61,538 Less valuation allowance (105,226 ) (47,053 ) Total deferred tax assets $ 14,899 $ 14,485 Deferred tax liabilities: Depreciation and amortization $ (6,815 ) $ (7,495 ) Right-of-use assets (7,934 ) (6,990 ) Others (150 ) — Gross deferred tax liabilities (14,899 ) (14,485 ) Deferred tax assets, net $ — $ — In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred assets will be realized. The ultimate realization of the deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Evaluating the need for a valuation allowance for deferred tax assets often requires judgment and analysis of all the positive and negative evidence available, including cumulative losses in recent years and projected future taxable income, to determine whether all or some portion of the deferred tax assets will not be realized. As of December 31, 2020, the Company has utilized a full valuation allowance to offset the net deferred tax assets as the Company believes it is not more likely than not that the net deferred tax assets will be fully realizable. The valuation allowance for deferred tax assets increased by $58.2 million during the year ended December 31, 2020. As of December 31, 2020, the Company had federal and state net operating loss (NOL) carryforwards of approximately $195.3 million and $191.4 million, respectively. Federal NOL carryforwards have an indefinite life and deductions cannot exceed 80% of taxable income. State NOL carryforwards will begin to expire as early as 2030, if not utilized, or have an indefinite life. As of December 31, 2020, the Company also had federal and California tax credit carryforward of approximately $14.4 million and $3.8 million, respectively. The federal tax credits will begin to expire in 2036 while the California tax credits have no expiration date. Generally, utilization of the NOL carryforwards and credits may be subject to an annual limitation due to the ownership change limitations provided by Section 382, which provides for limitations on NOL carryforwards and certain built-in losses following ownership changes, and Section 383, which provides for special limitations on certain excess credits, etc., of the Code, and similar state provisions. Accordingly, the Company’s ability to utilize NOL carryforwards may be limited as the result of such an “ownership change.” The carryforwards could be subject to an annual limitation, resulting in a reduction in the gross deferred tax assets before considering the valuation allowance. Further, a portion of the carryforwards may expire before being applied to reduce future earnings. The Company is not aware of any changes in ownership that would result in material limitations under Section 382 at this time. The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2020 and 2019 (in thousands): Balance as of December 31, 2018 $ 1,507 Decreases related to tax positions taken during the prior year (80 ) Increases related to tax positions taken during the current year 1,608 Balance as of December 31, 2019 3,035 Decreases related to tax positions taken during the prior year (446 ) Increases related to tax positions taken during the current year 2,003 Balance as of December 31, 2020 $ 4,592 If the unrecognized tax benefits for uncertain tax positions as of December 31, 2020, is recognized, there will be no impact to the effective tax rate as the tax benefit would increase the net deferred tax assets, which is currently offset with a full valuation allowance. The Company’s policy is to include interest and penalties related to unrecognized tax benefits, if any, within the provision for taxes in the consolidated statements of operations. The Company did not accrue any interest or penalties and does not have any tax positions for interest or penalties for the year ended December 31, 2020. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized tax benefits will significantly change within 12 months of December 31, 2020. The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. During the year ended December 31, 2020, the Company recorded a tax reserve of $2.0 million as a reduction of the tax credit carryover. The Company’s income tax returns generally remain subject to examination by federal and most state tax authorities. The Company is currently not subject to any income tax audits by federal or state taxing authorities. The statute of limitations for tax liabilities for all years remains open.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	9 . The Company has a collaboration agreement with Adimab, LLC (Adimab) under which the Company is developing antibodies discovered by Adimab in its AL001 and AL101 product candidates, and the Company is developing antibodies optimized by Adimab in its AL002 and AL003 product candidates (2014 Adimab Agreement). In August 2019, the Company signed a new collaboration agreement with Adimab for research and development of additional antibodies (2019 Adimab Agreement). The Chief Executive Officer of Adimab is a Co-founder and Chairperson of the board of directors of Alector. For the years ended December 31, 2020, 2019, and 2018, Alector incurred expenses of zero, $2.8 million, and $2.3 million for services provided by Adimab, respectively. The Company had no accrued liabilities due to Adimab as of December 31, 2020 and 2019. Under the 2014 Adimab Agreement, the Company will owe up to $3.5 million in milestone payments per program to Adimab for its product candidates. The Company will also owe low- to mid- single-digit royalty payments for commercial sales of such product candidates. Under the 2019 Adimab Agreement, the Company will owe certain milestone payments per program for its product candidates and low single-digit royalty payments for commercial sales of such product candidates. On January 30, 2020, in connection with the Company’s follow-on offering, a member of the Company’s board of directors purchased 20,000 shares of the Company’s common stock at the public offering price of $25.00 per share.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	10 . The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: Year Ended December 31, 2020 2019 2018 Convertible preferred stock — — 45,374,836 Restricted stock subject to future vesting 186,425 994,838 1,917,848 Options to purchase common stock 12,654,998 8,442,824 5,063,688 Shares committed under ESPP 86,677 44,464 — Total 12,928,100 9,482,126 52,356,372

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States (GAAP) as defined by the Financial Accounting Standards Board (FASB). The consolidated financial statements include the accounts of Alector, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expense during the reporting period. The Company evaluates its estimates, including those related to revenue recognition, manufacturing accruals, clinical accruals, fair value of assets and liabilities, income taxes uncertainties, stock-based compensation, and related 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 materially differ from those estimates .
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, and short-term marketable securities. Cash and cash equivalents are deposited in checking and sweep accounts at a financial institution. Such deposits may, at times, exceed federally insured limits.
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash and cash equivalents. Cash equivalents, which consist of amounts invested in money market funds, are stated at fair value. Restricted cash as of December 31, 2020 relates to a letter of credit established for a lease entered into in June 2018. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: Year Ended December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 49,969 $ 89,641 $ 65,470 Restricted cash 1,472 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 51,441 $ 91,113 $ 66,942
Marketable Securities	Marketable Securities All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. The Company considers its available-for-sale portfolio as available for use in current operations. Accordingly, the Company may classify certain investments as short-term marketable securities, even though the stated maturity date may be one year or more beyond the current balance sheet date. For available-for-sale debt securities, unrealized gains, net of any related tax effects, are excluded from earnings and are included in other comprehensive income and reported as a separate component of stockholders’ equity (deficit) until realized. The Company assesses available-for-sale debt securities on a quarterly basis to see if any unrealized loss is due to credit-related factors. Factors considered in determining whether an impairment is credit-related include the extent to which the investment’s fair value is less than its cost basis, declines in published credit ratings, changes in interest rates, and any other adverse factors related to the security. If it is determined that a credit-related impairment exists, the Company will measure the credit loss based on a discounted cash flows model. Credit-related impairments on available-for-sale debt securities are recognized as an allowance for credit losses with a corresponding adjustment to other income, net in the Company’s consolidated statement of operations. The unrealized loss position that is not credit-related is recorded, net of any related tax effects, in other comprehensive income until realized. There were no credit-related losses recognized for the periods presented. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. In accordance with our investment policy, management invests in money market funds, U.S. treasury securities, corporate bonds, commercial paper, and overnight repurchase agreements collateralized by U.S. treasury securities. The Company has not experienced any losses on its deposits of cash, cash equivalents, and marketable securities.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s believes the carrying value of financial instruments, including cash and cash equivalents, receivables, accounts payable, and accrued liabilities, approximate fair value due to their short-term nature. 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 the fair value of its financial instruments 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 are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.
Property, Plant and Equipment	Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the lesser of their useful lives or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statements of operations in the period realized. Maintenance and repairs are charged to the consolidated statements of operations as incurred.
Leases	Leases The Company adopted Accounting Standards Update No. 2016-02, Leases The Company determines whether an arrangement is or contains a lease at the inception of the lease. Leases are recognized on the balance sheet as right-of-use assets and lease liabilities. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and any prepaid or accrued rent. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company excludes balance sheet recognition of operating leases having a term of 12 months or less (short-term leases) and does not separate lease components and non-lease components for its long-term leases.
Impairment or Disposal of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If the total of the undiscounted future cash flows is less than the carrying amount of the assets, an impairment loss is recognized for the amount by which the carrying amount of the assets exceeds is fair value. For the year ended December 31, 2020, the Company recognized a $0.2 million impairment loss on its right-of-use asset related to decision to no longer utilize certain property through the end of the lease term. For the year ended December 31, 2019, the Company recognized a $1.2 million impairment loss on its right of use assets related to the portion of its headquarters being subleased.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when control of promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under arrangements, the Company performs the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies the performance obligation. Collaboration Revenue The Company signed an agreement in October 2017 with AbbVie Biotechnology, Ltd. (AbbVie) to co-develop antibodies to two program targets in preclinical development (AbbVie Agreement). Under the terms of the AbbVie Agreement, AbbVie made $205.0 million in upfront payments, of which $5.0 million and $200.0 million was received by the Company in October 2017 and January 2018, respectively. The Company will perform research and development services for the antibodies to the two programs through the end of Phase 2 clinical trials which the Company expects to conduct through 2023. AbbVie will then have the exclusive right to exercise an option to enter into a license and collaboration agreement with the Company for one or both of the programs for $250.0 million each. If AbbVie exercises its option for the programs, AbbVie will take over the development of the product candidates for such program and costs will be split between the parties. The Company will also share in profits and losses upon commercialization of any products from such program. However, following AbbVie’s exercise of its option for a program, the Company may opt out of sharing in development costs and profits or losses for that program and instead receive tiered royalties. Additionally, under the terms of the AbbVie Agreement, the Company will be eligible to earn up to an additional $242.8 million in milestone payments per program related to the initiation of certain clinical studies and regulatory approval for up to three indications per program. The Company assessed its collaboration agreement with AbbVie in the context of the delivery of the research and development services. The Company has determined that there are two research and development performance obligations as part of the agreement with AbbVie, one research and development performance obligation for each of the two research and development programs. The non-refundable upfront cash payments of $5.0 million and $200.0 million received in October 2017 and January 2018, respectively, were included in the transaction price. None of the remaining development and regulatory milestone and program opt-in payment amounts have been included in the transaction price, as all these amounts were fully constrained as of December 31, 2020 and 2019. As part of the Company’s evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestone amounts is outside the control of the Company and contingent upon success in future clinical trials. Any consideration related to royalties on net product sales will be recognized when the related sales occur and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company recognizes collaboration revenue by measuring the progress toward complete satisfaction of the performance obligation using an input measure. In order to recognize revenue over the research and development period, the Company measures actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligation. Revenues are recognized as the program costs are incurred. The Company will re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Clinical trials are expensive and can take many years to complete, and the outcome is inherently uncertain. Changes in the Company’s forecasted costs are likely to occur over time based upon changes in clinical trial procedures set forth in protocols, changes in estimates of manufacturing costs, or feedback from regulators on the design or operation of clinical trials. The Company has had changes to the overall expected costs to satisfy the performance obligations from period to period. For the year ended December 31, 2020, the Company had a 7% increase in the forecast of total expected costs due to changes to the clinical trial plans. For the year ended December 31, 2020, the increase in the overall expected costs to satisfy the performance obligation resulted in an approximately $6.7 million reduction in revenue compared to if the expected costs had remained the same, as a result of the cumulative catch up for the change in estimate. Collaboration revenue under the Company’s collaboration agreement with AbbVie during 2020, 2019, and 2018 was $21.1 million, $21.2 million, and $27.5 million, respectively. Collaboration revenue recognized during the year ended December 31, 2020, was included in deferred revenue at the beginning of the period. The balance of deferred revenue was $132.3 million as of December 31, 2020. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of Phase 2 clinical trials. The Company entered into an agreement in March 2020 with Innovent Biologics (Innovent) to license, develop, and commercialize AL008 in China (Innovent Agreement). AL008 is the Company’s novel antibody targeting the CD47-SIRP-alpha pathway, a potent survival pathway co-opted by tumors to evade the innate immune system. Under the terms of the Innovent Agreement, Innovent may pay the Company up to $11.5 million in development milestones, $112.5 million in sales milestones, and future royalties for any sales. The Company retains the rights to develop and commercialize AL008 outside of China. The Company has determined there is one performance obligation for the delivery of the license and will recognize revenue when it is probable that there will not be significant reversal of cumulative revenue. Development and sales milestones under the Innovent Agreement have not been included in the transaction price, as all these amounts were fully constrained as of December 31, 2020. As of December 31, 2020, no revenue has been recognized or payments received under the Innovent Agreement.
Research and Development Expense	Research and Development Costs Research and development costs are expensed as incurred and consist primarily of new product development. Research and development costs include salaries and benefits, consultants’ fees, process development costs, stock-based compensation, and laboratory supplies, as well as fees paid to third parties that conduct certain research and development activities on the Company’s behalf. In addition, research and development costs include the reimbursable costs incurred for the grant agreements, which includes payroll costs for time incurred on projects, laboratory supplies, and third-party research and development activities. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers. The Company records accrued expenses for estimated preclinical study and clinical trial expenses. Estimates are based on the services performed pursuant to contracts with research institutions, contract research organizations in connection with clinical studies, investigative sites in connection with clinical studies, vendors in connection with preclinical development activities, and contract manufacturing organizations in connection with the production of materials for clinical trials. Further, the Company accrues expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. The Company monitors patient enrollment levels and related activity to the extent reasonably possible and make judgments and estimates in determining the accrued balance in each reporting period. If the Company underestimates or overestimates the level of services performed or the costs of these services, actual expenses could differ from estimates. To date, the Company has not experienced significant changes in its estimates of preclinical studies and clinical trial accruals.
Stock-based Compensation	Stock-based Compensation Stock-based compensation for employee awards is measured on the grant date based on the fair value of the award and recognized on a straight-line basis over the requisite service period. The fair value of options to purchase common stock are measured using the Black-Scholes option-pricing model. The Company accounts for forfeitures as they occur.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are the result of transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive loss was net unrealized gain (loss) on marketable securities.
Income Tax	Income Taxes The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have The deferred tax assets are recognized to the extent the Company believes that these assets are more likely than not to be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior periods, the net deferred tax assets have been fully offset by a valuation allowance. The Company records uncertain tax positions using a two-step process. First, the Company determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position. Second, for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits on the interest expense line and other expense line, respectively, in the accompanying statements of operations. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Employee 401 (K) Plan	Employee 401(k) Plan The Company has a qualified contributory savings plan under Section 401(k) of the Internal Revenue Code (the “Code”) covering substantially all U.S. employees of Alector. The 401(k) plan is designed to provide tax-deferred retirement benefits in accordance with the provisions of Section 401(k) of the Code. Eligible employees may defer up to 100% of their eligible compensation up to the annual maximum as determined by the Internal Revenue Service. The Company’s contributions to the plan are discretionary. For the year ended December 31, 2020, the Company made matching contributions of $0.7 million. The Company did not make any matching contributions for the years ended December 31, 2019 or 2018.
Segment	Segments The Company operates in one segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources.
New Accounting Pronouncements	Recent Accounting Pronouncements Effective January 1, 2020, the Company adopted Accounting Standards Update (ASU) No. 2016-13, Financial Instruments—Credit Losses (Topic 326) In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740) Simplifying the Accounting for Income Taxes.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: Year Ended December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 49,969 $ 89,641 $ 65,470 Restricted cash 1,472 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 51,441 $ 91,113 $ 66,942

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on a Recurring Basis	The following tables summarize the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2020 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 37,951 $ — $ — $ 37,951 U.S. government treasury securities Level 1 357,725 620 (6 ) 358,339 Corporate bonds Level 2 5,000 — — 5,000 Total cash equivalents and marketable securities $ 400,676 $ 620 $ (6 ) $ 401,290 December 31, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 61,104 $ — $ — $ 61,104 U.S. government treasury securities Level 1 227,446 149 (8 ) 227,587 Commercial paper Level 2 43,735 — — 43,735 Corporate bonds Level 2 10,103 3 (2 ) 10,104 Total cash equivalents and marketable securities $ 342,388 $ 152 $ (10 ) $ 342,530

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2020 2019 (In thousands) Leasehold improvements $ 25,542 $ 25,222 Lab equipment 11,610 10,458 Furniture and fixtures 2,222 2,031 Computer equipment 1,764 1,554 Property and equipment, gross 41,138 39,265 Less accumulated depreciation and amortization (10,957 ) (5,413 ) Total property and equipment, net $ 30,181 $ 33,852
Summary of Accrued Liabilities	Accrued liabilities consist of the following: December 31, 2020 2019 (In thousands) Accrued research and development costs $ 13,375 $ 6,400 Accrued employee compensation 7,288 5,730 Accrued professional services 1,094 2,040 Accrued property and equipment 428 3,471 Other 353 615 Total accrued liabilities $ 22,538 $ 18,256

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Components of Lease Expense	The components of lease expense were as follows: December 31, 2020 2019 (In thousands) Operating lease cost $ 5,589 $ 5,875 Variable lease cost 2,255 1,328 Short-term lease cost — 68 Sublease income and reimbursement of variable lease cost (2,288 ) (1,263 ) Total $ 5,556 $ 6,008
Schedule of Lease Payments Owed Under Company's Operating Leases	The following are the lease payments owed under the Company’s operating leases as of December 31, 2020: (In thousands) 2021 $ 7,872 2022 8,171 2023 8,436 2024 8,732 2025 9,037 Thereafter 30,130 Total undiscounted lease payments $ 72,378 Less: Present value adjustment (21,122 ) Total lease liability $ 51,256

Stock-based Compensation (Table

Stock-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Recognized Stock-Based Compensation	The Company recognized stock-based compensation as follows: Year Ended December 31, 2020 2019 2018 (In thousands) Research and development $ 14,980 $ 8,031 $ 4,195 General and administrative 15,542 8,250 2,730 Total stock-based compensation $ 30,522 $ 16,281 $ 6,925
Summary of Share-based Payment Award, Stock Options, Valuation Assumptions	The estimated grant-date fair value of all the Company’s options to purchase common stock was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2020 2019 2018 Expected term (in years) 5.3-6.1 5.3 – 6.1 5.1 – 6.1 Expected volatility 74%-78% 76% – 82% 73% – 81% Risk-free interest rate 0.3% - 1.8% 1.4% – 2.6% 2.5% – 3.1% Dividend yield 0% 0% 0%
Summary of Restricted Common Stock	Activity for the restricted common stock is shown below: Number of Shares Weighted Average Grant Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 1,917,848 $ 6.95 Vested (866,037 ) 6.95 Forfeited (56,973 ) 6.95 Unvested restricted common stock as of December 31, 2019 994,838 6.95 Vested (726,659 ) 6.95 Forfeited (81,754 ) 6.95 Unvested restricted common stock as of December 31, 2020 186,425 6.95
Summary of Options to Purchase Common Stock	Activity for the options to purchase common stock shown below: Number of Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In years) (In thousands) Outstanding as of December 31, 2018 5,063,688 $ 8.94 Granted 3,738,071 17.74 Exercised (180,287 ) 8.43 Forfeited (178,648 ) 11.52 Outstanding as of December 31, 2019 8,442,824 12.79 Granted 5,597,148 15.35 Exercised (655,772 ) 9.65 Forfeited (729,202 ) 14.96 Outstanding as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633 Exercisable as of December 31, 2020 3,588,568 $ 12.74 8.1 $ 13,393 Vested and expected to vest as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2020, 2019, and 2018 (in thousands): Year Ended December 31, 2020 2019 2018 Tax benefit at federal statutory rate $ (39,948 ) $ (22,081 ) $ (10,972 ) State income taxes (16,570 ) (4,694 ) — Tax credits (8,010 ) (6,530 ) (2,387 ) Uncertain tax positions 2,003 2,044 708 Stock-based compensation 353 1,493 1,251 Nondeductible expense 51 124 58 Tax benefit recognized on utilization of attributes previous reserved — (13,582 ) — Deferred adjustment from amended tax return — (1,073 ) — Change in valuation allowance 62,280 44,748 11,346 Others (159 ) (449 ) (4 ) Income tax provision $ — $ — $ —
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets consist of (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss $ 54,382 $ 2,223 Accrued bonus 1,201 370 Tax credits 13,000 7,319 Stock-based compensation 6,493 2,015 Deferred revenue 32,129 37,621 Lease liability 12,457 11,781 Others 463 209 Gross deferred tax assets 120,125 61,538 Less valuation allowance (105,226 ) (47,053 ) Total deferred tax assets $ 14,899 $ 14,485 Deferred tax liabilities: Depreciation and amortization $ (6,815 ) $ (7,495 ) Right-of-use assets (7,934 ) (6,990 ) Others (150 ) — Gross deferred tax liabilities (14,899 ) (14,485 ) Deferred tax assets, net $ — $ —
Schedule of Unrecognized Tax Benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2020 and 2019 (in thousands): Balance as of December 31, 2018 $ 1,507 Decreases related to tax positions taken during the prior year (80 ) Increases related to tax positions taken during the current year 1,608 Balance as of December 31, 2019 3,035 Decreases related to tax positions taken during the prior year (446 ) Increases related to tax positions taken during the current year 2,003 Balance as of December 31, 2020 $ 4,592

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: Year Ended December 31, 2020 2019 2018 Convertible preferred stock — — 45,374,836 Restricted stock subject to future vesting 186,425 994,838 1,917,848 Options to purchase common stock 12,654,998 8,442,824 5,063,688 Shares committed under ESPP 86,677 44,464 — Total 12,928,100 9,482,126 52,356,372

The Company and Liquidity - Add

The Company and Liquidity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 30, 2020	Feb. 07, 2019	Dec. 31, 2020	Dec. 31, 2019
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings			9,602,500	9,739,541
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings		9,739,541
Share price		$ 19
Proceeds from issuance of common stock, net of underwriting discounts and commissions and offering expenses		$ 168.2
Convertible preferred stock, shares outstanding		0
Initial Public Offering | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Conversion of convertible preferred stock to common stock		45,374,836
Over-Allotment Option
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	1,252,500	489,541
Follow-on-offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	9,602,500
Share price	$ 25
Proceeds from issuance of common stock, net of underwriting discounts and commissions and offering expenses	$ 224.5

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Credit losses on marketable securities			$ 0	$ 0
Impairment loss on right-of-use assets			238,000	1,158,000
Deferred revenue			$ 132,300,000
Increase in the forecast of total expected costs			7.00%
Reduction in revenue due to increase in budget during the year			$ 6,700,000
Defined contribution plan, maximum annual contributions per employee, percent			100.00%
Defined matching contribution plan			$ 700,000	0	$ 0
AbbVie Biotechnology Limited
Summary Of Significant Accounting Policies [Line Items]
Upfront payments		$ 205,000,000
Upfront payment received	$ 200,000,000	5,000,000
Exclusive option rights exercised for each program		250,000,000
Cash payment received, non-refundable	$ 200,000,000	5,000,000
AbbVie Biotechnology Limited | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue			21,100,000	$ 21,200,000	$ 27,500,000
Innovent
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue			0
Development milestone			11,500,000
Sales milestone			$ 112,500,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life			3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life			5 years
Maximum | AbbVie Biotechnology Limited
Summary Of Significant Accounting Policies [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval		$ 242,800,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 49,969	$ 89,641	$ 65,470
Restricted cash	1,472	1,472	1,472
Total cash, cash equivalents, and restricted cash	$ 51,441	$ 91,113	$ 66,942	$ 32,451

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	$ 37,951	$ 61,104
Cash equivalents and marketable securities, Fair Market Value	37,951	61,104
U.S Government Treasury Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	357,725	227,446
Cash equivalents and marketable securities, Unrealized Gains	620	149
Cash equivalents and marketable securities, Unrealized Losses	(6)	(8)
Cash equivalents and marketable securities, Fair Market Value	358,339	227,587
Corporate Bonds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	5,000	10,103
Cash equivalents and marketable securities, Unrealized Gains		3
Cash equivalents and marketable securities, Unrealized Losses		(2)
Cash equivalents and marketable securities, Fair Market Value	5,000	10,104
Cash Equivalents and Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	400,676	342,388
Cash equivalents and marketable securities, Unrealized Gains	620	152
Cash equivalents and marketable securities, Unrealized Losses	(6)	(10)
Cash equivalents and marketable securities, Fair Market Value	$ 401,290	342,530
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		43,735
Cash equivalents and marketable securities, Fair Market Value		$ 43,735

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) $ in Thousands	Dec. 31, 2020USD ($)
Fair Value Disclosures [Abstract]
Fair value of assets transferred from level 1 to level 2	$ 0
Fair value of assets transferred from level 2 to level 1	0
Remaining contractual maturities of investments were less than one year	336,900
Remaining contractual maturities of investments were less than one year through two years	$ 64,400

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 41,138	$ 39,265
Less accumulated depreciation and amortization	(10,957)	(5,413)
Total property and equipment, net	30,181	33,852
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	11,610	10,458
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,764	1,554
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	25,542	25,222
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,222	$ 2,031

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued research and development costs	$ 13,375	$ 6,400
Accrued employee compensation	7,288	5,730
Accrued professional services	1,094	2,040
Accrued property and equipment	428	3,471
Other	353	615
Total accrued liabilities	$ 22,538	$ 18,256

Leases - Additional Information

Leases - Additional Information (Detail)	1 Months Ended				12 Months Ended
	Dec. 31, 2020USD ($)ft²	Oct. 31, 2020USD ($)ft²	May 31, 2019ft²	Jun. 30, 2018ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Tenant improvement allowance					$ 15,700,000
Amount form letter of credit	$ 1,472,000				$ 1,472,000	$ 1,472,000	$ 1,472,000
Lease expiration year					2029
Operating lease, renewal period	10 years				10 years
Operating lease term					The lease expires in 2029 with an option to renew for a period of ten years
Operating sublease, impairment charges					$ 238,000	1,158,000
Rent expense							$ 1,100,000
Weighted average remaining lease term for operating leases	8 years 2 months 12 days				8 years 2 months 12 days
Weighted average discount rate for operating leases	8.50%				8.50%
Cash paid for amounts included In measurement of lease liabilities					$ 6,900,000	$ 3,200,000
Offices and Laboratory Facilities | South San Francisco
Lessee Lease Description [Line Items]
Area under lease | ft²	23,600			105,000
Area under sublease | ft²			25,000
Sublease expiration month and year					2021-11
Operating sublease, option to extend					The sublessor has the option to extend the Sublease for one additional year subject to the consent of the Company.
Sublease renewal period			1 year
Operating sublease, existence of option to extend					true
Ongoing loss over sublease period					$ 0
Offices and Laboratory Facilities | South San Francisco | General and Administrative Expense
Lessee Lease Description [Line Items]
Operating sublease, impairment charges					$ 1,200,000
Offices and Laboratory Facilities | South San Francisco
Lessee Lease Description [Line Items]
Tenant improvement allowance		$ 400,000
Area of property | ft²		18,700
Lease, description					In October 2020, the Company signed a lease agreement to lease approximately 18,700 square feet of office and laboratory space in Newark, California. The lease term ends on February 6, 2028 with an option to extend for an additional five years. The landlord is obligated to pay for up to $0.4 million of tenant improvements
Lease expiration date		Feb. 6, 2028
Extend for an additional lease term		5 years

Leases - Components of Lease Ex

Leases - Components of Lease Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 5,589	$ 5,875
Variable lease cost	2,255	1,328
Short-term lease cost		68
Sublease income and reimbursement of variable lease cost	2,288	1,263
Total	$ 5,556	$ 6,008

Leases - Schedule of Lease Paym

Leases - Schedule of Lease Payments Owed Under Company's Operating Leases (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 7,872
2022	8,171
2023	8,436
2024	8,732
2025	9,037
Thereafter	30,130
Total undiscounted lease payments	72,378
Less: Present value adjustment	(21,122)
Total lease liability	$ 51,256

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 30,522	$ 16,281	$ 6,925
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	14,980	8,031	4,195
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 15,542	$ 8,250	$ 2,730

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions (Details) - Share-based Payment Arrangement, Option	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 3 months 18 days	5 years 1 month 6 days
Expected volatility	74.00%	76.00%	73.00%
Risk-free interest rate	0.30%	1.40%	2.50%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Expected volatility	78.00%	82.00%	81.00%
Risk-free interest rate	1.80%	2.60%	3.10%

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 06, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Expected weighted average period		2 years 7 months 6 days
Common stock reserved for issuance		15,628,217
Percentage of stock owned by Individual	10.00%
Percentage of exercise price per share from fair market value	110.00%
Stock options, term	5 years
Cliff vesting percentage	25.00%
Cliff vesting period	1 year
Common stock, shares available for issuance	2,973,219
Aggregate intrinsic value of option exercised		$ 11.1	$ 1.8	$ 0
Unrecognized stock-based compensation		$ 82.9
Weighted-average grant-date fair value per share of options granted		$ 10.14	$ 12.06	$ 6.27
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of common stock, percentage	85.00%
Subsequent offering period	6 months
Un-recognized compensation expense		$ 0.4
Minimum | 2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, term	10 years
Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage		25.00%
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount		$ 0.9
Expected weighted average period		9 months 18 days

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Restricted Common Stock (Details) - Restricted Common Stock - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Shares
Number of Shares, Unvested, beginning balance	994,838	1,917,848
Number of Shares, Vested	(726,659)	(866,037)
Number of Shares, Forfeited	(81,754)	(56,973)
Number of Shares, Unvested, ending balance	186,425	994,838
Weighted-average grant-date fair value
Weighted Average Grant Date Fair Value per Share, Unvested, beginning balance	$ 6.95	$ 6.95
Weighted Average Grant Date Fair Value per Share, Vested	6.95	6.95
Weighted Average Grant Date Fair Value per Share, Forfeited	6.95	6.95
Weighted Average Grant Date Fair Value per Share, Unvested, ending balanc	$ 6.95	$ 6.95

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Options to Purchase Common Stock (Details) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance	8,442,824	5,063,688
Number of Options, Granted	5,597,148	3,738,071
Number of Options, Exercised	(655,772)	(180,287)
Number of Options, Forfeited	(729,202)	(178,648)
Number of Options, Outstanding, ending balance	12,654,998	8,442,824	5,063,688
Number of Options, Exercisable	3,588,568
Number of Options, Vested and expected to vest	12,654,998
Weighted average exercise price, Outstanding, beginning balance	$ 12.79	$ 8.94
Weighted average exercise price, Granted	15.35	17.74
Weighted average exercise price, Exercised	9.65	8.43
Weighted average exercise price, Forfeited	14.96	11.52
Weighted average exercise price, Outstanding, ending balance	13.96	$ 12.79	$ 8.94
Weighted average exercise price, Exercisable	12.74
Weighted average exercise price, Vested and expected to vest	$ 13.96
Weighted Average Remaining Contractual Term, Outstanding	8 years 8 months 12 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	8 years 1 month 6 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	8 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 39,633	$ 0	$ 0
Aggregate Intrinsic Value, Exercised			$ 0
Aggregate Intrinsic Value, Exercisable	13,393
Aggregate Intrinsic Value, Vested or expected to vest	$ 39,633

Schedule of Effective Income Ta

Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ (39,948)	$ (22,081)	$ (10,972)
State income taxes	(16,570)	(4,694)
Tax credits	(8,010)	(6,530)	(2,387)
Uncertain tax positions	2,003	2,044	708
Stock-based compensation	353	1,493	1,251
Nondeductible expense	51	124	58
Tax benefit recognized on utilization of attributes previous reserved		(13,582)
Deferred adjustment from amended tax return		(1,073)
Change in valuation allowance	62,280	44,748	11,346
Others	$ (159)	$ (449)	$ (4)

Schedule of Deferred Tax Assets

Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss	$ 54,382	$ 2,223
Accrued bonus	1,201	370
Tax credits	13,000	7,319
Stock-based compensation	6,493	2,015
Deferred revenue	32,129	37,621
Lease liability	12,457	11,781
Others	463	209
Gross deferred tax assets	120,125	61,538
Less valuation allowance	(105,226)	(47,053)
Total deferred tax assets	14,899	14,485
Deferred tax liabilities:
Depreciation and amortization	(6,815)	(7,495)
Right-of-use assets	(7,934)	(6,990)
Others	(150)
Gross deferred tax liabilities	$ (14,899)	$ (14,485)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Income Tax Contingency [Line Items]
Valuation allowance deferred tax asset change in amount	$ 58.2
Tax Credit Carryforward, Amount	2
California Tax Authority
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount	3.8
Federal and State Tax Authorities
Income Tax Contingency [Line Items]
Operating loss carry forward, net	195.3
Federal Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward, net	191.4
Tax Credit Carryforward, Amount	$ 14.4
Tax Credit Beginning Expiration Year	2036
Federal Tax Authority | Maximum
Income Tax Contingency [Line Items]
Operating loss carry forward, deductions	80.00%
State Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward beginning expiration year	2030

Schedule of Unrecognized Tax Be

Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 3,035	$ 1,507
Decreases related to tax positions taken during the prior year	(446)	(80)
Increases related to tax positions taken during the current year	2,003	1,608
Unrecognized tax benefits, ending balance	$ 4,592	$ 3,035

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	Jan. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Follow-on-offering
Related Party Transaction [Line Items]
Public offerings	9,602,500
Share price	$ 25
Follow-on-offering | Director
Related Party Transaction [Line Items]
Public offerings	20,000
Share price	$ 25
Adimab
Related Party Transaction [Line Items]
Related party transaction expenses		$ 0	$ 2,800,000	$ 2,300,000
Accrued liabilities due to related parties		0	$ 0
Two Thousand Fourteen Adimab Agreement | Maximum
Related Party Transaction [Line Items]
Future milestone payments owed		$ 3,500,000

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	12,928,100	9,482,126	52,356,372
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares			45,374,836
Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	186,425	994,838	1,917,848
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	12,654,998	8,442,824	5,063,688
Shares Committed Under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	86,677	44,464