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Stock-based Compensation - Summary of Share-based Payment Award, Stock Options, Valuation Assumptions (Details) - Employee Stock Option	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years
Expected volatility, Minimum	81.00%	79.00%	79.00%
Expected volatility, Maximum		80.00%	80.00%
Risk free interest rate, Minimum	4.30%	3.50%	1.50%
Risk free interest rate, Maximum		4.60%	4.30%
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
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		6 years	6 years 1 month 6 days

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 39,463	$ 42,798	$ 46,146
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	19,836	22,407	24,895
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 19,627	$ 20,391	$ 21,251

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

Fair Value Measurements

12 Months Ended

Dec. 31, 2024

Fair Value Disclosures [Abstract]

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, 2024
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	31,310	$	—	$	—		$	31,310
U.S. government treasury securities	Level 1		72,360		38		(38	)		72,360
Certificates of deposit	Level 2		15,955		4		(1	)		15,958
Commercial paper	Level 2		81,744		51		(15	)		81,780
Corporate bonds	Level 2		210,056		301		(79	)		210,278
Total cash equivalents and marketable securities		$	411,425	$	394	$	(133	)	$	411,686

	December 31, 2023
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	67,101	$	—	$	—		$	67,101
U.S. government treasury securities	Level 1		178,232		86		(112	)	$	178,206
Certificates of deposit	Level 2		29,086		63		—		$	29,149
Commercial paper	Level 2		140,082		85		(34	)		140,133
Corporate bonds	Level 2		129,474		173		(78	)		129,569
Total cash equivalents and marketable securities		$	543,975	$	407	$	(224	)	$	544,158

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. The Company classifies marketable securities available to fund current operations as current assets. As of December 31, 2024, the remaining contractual maturities of $364.8 million of investments were less than one year and $46.9 million of investment were after one year through two years. The Company does not intend to sell the investments that are currently in an unrealized loss position, and it is highly unlikely that the Company will be required to sell the investments before recovery of their amortized cost basis, which may be at maturity. For the years ended December 31, 2024 and 2023, the Company sold marketable securities for the total proceeds of $2.6 million and $3.5 million for immaterial realized losses or gains based on the specific identification method. The Company did not sell any marketable securities for the year ended December 31, 2022.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

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+ Details

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2024	Dec. 31, 2023
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	99,085,888	84,879,693
Common stock, shares outstanding	99,085,888	84,879,693

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities:
Net loss	$ (119,049)	$ (130,391)	$ (133,310)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,407	5,725	5,714
Stock-based compensation	39,463	42,798	46,146
Amortization of premiums and accretion of discounts on marketable securities	(14,379)	(15,318)	(1,163)
Amortization of right-of-use assets	3,434	3,123	2,758
Impairment loss of right-of-use assets and leasehold improvements	2,205	0	0
Loss from disposal of fixed assets	0	48	0
Changes in operating assets and liabilities:
Receivable from collaboration partner	0	2,587	4,804
Prepaid expenses and other current assets	5,526	(5,949)	(3,926)
Other assets	2,874	(1,009)	(835)
Accounts payable	(1,441)	(377)	(572)
Accrued liabilities and accrued clinical supply costs	340	2,546	(2,655)
Payable to collaboration partner	(1,789)	7,703	0
Deferred revenue	(100,558)	(66,782)	66,383
Refund liability to collaboration partner	(46,007)	(24,512)	0
Lease liabilities	(5,792)	(4,968)	(4,274)
Other long-term liabilities	(139)	614	601
Net cash used in operating activities	(229,905)	(184,162)	(20,329)
Cash flows from investing activities:
Purchase of property and equipment	(1,255)	(2,381)	(4,117)
Purchase of marketable securities	(467,741)	(551,729)	(556,898)
Maturities of marketable securities	573,559	652,523	402,001
Sale of marketable securities	2,568	3,505	0
Net cash provided by (used in) investing activities	107,131	101,918	(159,014)
Cash flows from financing activities:
Proceeds from the exercise of options to purchase common stock	0	1,079	3,059
Proceeds from issuance of stock from employee stock purchase plan	1,043	1,471	1,455
Proceeds from issuance of common stock upon public offering, net of issuance costs	71,108	0	0
Proceeds from debt financing, net of discount and issuance costs	9,389	0	0
Net cash provided by financing activities	81,540	2,550	4,514
Net decrease in cash, cash equivalents, and restricted cash	(41,234)	(79,694)	(174,829)
Cash, cash equivalents, and restricted cash at beginning of period	76,101	155,795	330,624
Cash, cash equivalents, and restricted cash at end of period	34,867	76,101	155,795
Non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	$ 0	$ 172	$ 493

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2024

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,
	2024		2023		2022
Restricted stock subject to future vesting		8,365,063		6,092,101		3,044,618
Options to purchase common stock		10,723,681		12,096,237		13,713,272
Shares committed under 2019 ESPP		146,316		160,356		167,133
Total		19,235,060		18,348,694		16,925,023

Debt

12 Months Ended

Dec. 31, 2024

Debt Disclosure [Abstract]

Debt

10. Debt

On November 14, 2024 (the Closing Date), the Company entered into a loan and security agreement (the Loan Agreement) with its subsidiary Alector LLC as a co-borrower (together with the Company), several banks and other financial institutions from time to time party thereto (collectively, the Lenders) and Hercules Capital,

Inc., as administrative agent and collateral agent. The Loan Agreement provides for a senior secured term loan facility in an aggregate principal amount of up to $50.0 million, available in up to two tranches (the Term Loans). The initial tranche of Term Loans in an aggregate principal amount of up to $25.0 million is available through June 30, 2026, subject to the satisfaction of applicable conditions set forth in the Loan Agreement. The second tranche of Term Loans in an aggregate principal amount of up to $25.0 million is available at the sole discretion of the Lenders.

Borrowings under the Loan Agreement accrue interest at a rate equal to the greater of (A) the prime rate plus 1.05% and (B) 8.05%. The Term Loans are repayable in monthly interest-only payments until December 1, 2026 (the Interest-Only Payment Period). The Interest-Only Payment Period may be extended by up to twenty-four (24) months, subject to the achievement by the Company of certain milestones as set forth in the Loan Agreement. After the expiration of the Interest-Only Payment Period, the Term Loans are repayable in equal monthly payments of principal and accrued interest until maturity. The Term Loans will mature on December 1, 2028 (the Maturity Date). At the Company’s option, the Company may prepay all or a portion of the outstanding Term Loans, subject to a prepayment premium equal to (a) 2.0% of the Term Loans being prepaid if the prepayment occurs during the 12 months following the Closing Date; (b) 1.5% of the Term Loans being prepaid if the prepayment occurs after the 12 month anniversary of the Closing Date but on or prior to the 24 month anniversary of the Closing Date; and (c) 0.5% of the Term Loans being prepaid if the prepayment occurs after 24 months following the Closing Date and prior to the Maturity Date. In addition, the Company will pay an end of term charge of (i) 2.45% if the Term Loans are prepaid or repaid within the first 24 months of the Closing Date; or (ii) 4.75% if the Term Loans are prepaid or repaid after 24 months from the Closing Date (including on the Maturity Date). The Company paid an initial facility charge of $250,000 on the Closing Date, and thereafter, the Company will pay a facility charge of 1.00% upon any draw of the Term Loans under the second tranche. The Company’s obligations under the Loan Agreement are secured by substantially all of the Company’s assets, including intellectual property, subject to certain exceptions, including exceptions with respect to assets and intellectual property subject to the Company’s existing agreements with each of Adimab LLC and Glaxo Wellcome UK Limited, and previously subject to the agreement with AbbVie Biotechnology, Ltd.

The Loan Agreement contains customary affirmative and negative covenants, including covenants limiting the ability of the Company and their subsidiaries to, among other things, dispose of assets, enter into certain licensing arrangements, effect certain mergers, incur debt, grant liens, pay dividends and distributions on their capital stock, make investments and acquisitions, and enter into transactions with affiliates, in each case subject to customary exceptions for a loan facility of this size and type. The Loan Agreement also includes customary events of default, including, among others, payment defaults, material misrepresentations, breaches of covenants following any applicable cure period, cross defaults with certain other indebtedness or material agreements, bankruptcy and insolvency events, judgment defaults and the occurrence of certain events that could reasonably be expected to have a “material adverse effect.” The occurrence of an event of default could result in the acceleration of the Company’s obligations under the Loan Agreement, the termination of the Lenders’ commitments, a 5% increase in the applicable rate of interest and the exercise by Agent of other rights and remedies provided for under the Loan Agreement.

On November 14, 2024, the Company drew down $10 million from the initial tranche and received proceeds of approximately $9.6 million after charges payable to Hercules Capital. The Company incurred $0.4 million in debt issuance costs in relation to the initial draw. As of December 31, 2024, the face value of the outstanding balance of the Term Loan was $10 million, less unamortized discount and unaccreted value of the End of Term Charge of $0.6 million based on the imputed interest rate of 12.2%. The fair value of the Term Loan as of December 31, 2024 is a Level 3 measurement considered to approximate its carrying value of $9.4 million due to the short period of time since the Term Loan was entered into and the interest rates upon which the terms of the Term Loan were based, notably the Prime Rate, have not changed significantly since the Term Loan was drawn.

The aggregate maturity of the term loan for the next four years from December 31, 2024 is as follows:


	Maturity
2025	$	—
2026		365,914
2027		4,596,616
2028		5,037,470
Total Principal repayments	$	10,000,000

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
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-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (a)(1)
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Paragraph 1B
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-URI https://asc.fasb.org/470/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (g)
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 1B
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

+ Details

Debt - Schedule of Maturities of Term Loan (Details)	Dec. 31, 2024 USD ($)
Debt Disclosure [Abstract]
2025	$ 0
2026	365,914
2027	4,596,616
2028	5,037,470
Total Principal repayments	$ 10,000,000

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Debt (Tables)

12 Months Ended

Dec. 31, 2024

Debt Disclosure [Abstract]

Schedule of Maturities of Term Loan

The aggregate maturity of the term loan for the next four years from December 31, 2024 is as follows:


	Maturity
2025	$	—
2026		365,914
2027		4,596,616
2028		5,037,470
Total Principal repayments	$	10,000,000

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Sep. 22, 2022	Feb. 06, 2019	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jan. 01, 2024	Jan. 01, 2022
Restricted Stock Awards And Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected weighted average period			2 years 2 months 12 days
Grant date fair value of units granted			$ 6.6
Share-based payment award requisite service period			2 years 1 month 6 days
Un-recognized compensation expense			$ 42.2
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			1 year 6 months
Automatic conversion provision of common stock reserved for issuance			24,364,669			4,243,984
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			6,781,827
Aggregate intrinsic value of option exercised			$ 0.0	$ 0.1	$ 1.2
Unrecognized stock-based compensation			$ 13.8
Weighted-average grant-date fair value per share of options granted			$ 4.45	$ 4.93	$ 8.42
2019 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, term		10 years
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Automatic conversion provision of common stock reserved for issuance						591,397
Purchase price of common stock, percentage			85.00%
Subsequent offering period			6 months
Un-recognized compensation expense			$ 0.1
2022 Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Automatic conversion provision of common stock reserved for issuance							1,630,000
Common stock, shares available for issuance			1,547,349
Number of additional shares authorized (in shares)	3,300,000

Income Taxes - Schedule of Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 20,204,000	$ 15,138,000
Decreases related to tax positions taken during the prior year	0	(594,000)
Increase related to tax positions taken during the prior year	11,000	2,588,000
Increases related to tax positions taken during the current year	2,710,000	3,072,000
Unrecognized tax benefits, ending balance	$ 22,925,000	$ 20,204,000

The Company and Liquidity

12 Months Ended

Dec. 31, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The Company and Liquidity

Alector, Inc. (Alector or the Company) is a Delaware corporation headquartered in South San Francisco, California. Alector is a late-stage clinical biotechnology company focused on developing therapies to counteract the devastating progression of neurodegeneration.

Public Offering

On January 19, 2024, the Company completed a public offering through selling and issuing 10,869,566 shares of common stock at a price per share of $6.57, resulting in aggregate net proceeds of $71.1 million, after deducting underwriting discounts and commissions and offering costs.

Net Loss Per Share

12 Months Ended

Dec. 31, 2024

Earnings Per Share [Abstract]

Net Loss Per Share

11. 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,
	2024		2023		2022
Restricted stock subject to future vesting		8,365,063		6,092,101		3,044,618
Options to purchase common stock		10,723,681		12,096,237		13,713,272
Shares committed under 2019 ESPP		146,316		160,356		167,133
Total		19,235,060		18,348,694		16,925,023

Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance cost of common stock	$ 3,892

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2024

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, 2024
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	31,310	$	—	$	—		$	31,310
U.S. government treasury securities	Level 1		72,360		38		(38	)		72,360
Certificates of deposit	Level 2		15,955		4		(1	)		15,958
Commercial paper	Level 2		81,744		51		(15	)		81,780
Corporate bonds	Level 2		210,056		301		(79	)		210,278
Total cash equivalents and marketable securities		$	411,425	$	394	$	(133	)	$	411,686

	December 31, 2023
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	67,101	$	—	$	—		$	67,101
U.S. government treasury securities	Level 1		178,232		86		(112	)	$	178,206
Certificates of deposit	Level 2		29,086		63		—		$	29,149
Commercial paper	Level 2		140,082		85		(34	)		140,133
Corporate bonds	Level 2		129,474		173		(78	)		129,569
Total cash equivalents and marketable securities		$	543,975	$	407	$	(224	)	$	544,158

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2021		81,986,192
Beginning Balance at Dec. 31, 2021	$ 300,724	$ 8	$ 748,036	$ (943)	$ (446,377)
Exercise of stock options (in shares)		298,238
Exercise of stock options	3,059		3,059
Vesting of restricted stock units		414,578
Purchase of common stock under employee stock purchase plan (in shares)		196,710
Purchase of common stock under employee stock purchase plan	1,455		1,455
Stock-based compensation	46,146		46,146
Unrealized gain (loss) on marketable securities	(3,632)			(3,632)
Net Income (Loss)	(133,310)				(133,310)
Ending balance (in shares) at Dec. 31, 2022		82,895,718
Ending Balance at Dec. 31, 2022	214,442	$ 8	798,696	(4,575)	(579,687)
Exercise of stock options (in shares)		132,191
Exercise of stock options	1,079		1,079
Vesting of restricted stock units		1,584,449
Purchase of common stock under employee stock purchase plan (in shares)		267,335
Purchase of common stock under employee stock purchase plan	1,471		1,471
Stock-based compensation	42,798		42,798
Unrealized gain (loss) on marketable securities	4,759			4,759
Net Income (Loss)	$ (130,391)				(130,391)
Ending balance (in shares) at Dec. 31, 2023	84,879,693	84,879,693
Ending Balance at Dec. 31, 2023	$ 134,158	$ 8	844,044	184	(710,078)
Issuance of common stock upon public offering, net of issuance cost $3,892 (in shares)		10,869,566
Issuance of common stock upon public offering, net of issuance cost $3,892	71,108	$ 1	71,107
Vesting of restricted stock units		3,001,132
Purchase of common stock under employee stock purchase plan (in shares)		335,497
Purchase of common stock under employee stock purchase plan	1,043		1,043
Stock-based compensation	39,463		39,463
Unrealized gain (loss) on marketable securities	77			77
Net Income (Loss)	$ (119,049)				(119,049)
Ending balance (in shares) at Dec. 31, 2024	99,085,888	99,085,888
Ending Balance at Dec. 31, 2024	$ 126,800	$ 9	$ 955,657	$ 261	$ (829,127)

Leases

12 Months Ended

Dec. 31, 2024

Leases [Abstract]

Leases

7. Leases

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 November 2021, the Company entered into an agreement to sublease approximately 7,100 square feet of the Headquarters, which was amended in May 2022 to include an additional space of 13,150 square feet. The 7,100 square feet of space subleased expired in December 2022 and the additional space of 13,150 expired in

November 2023. In October 2023, the Company entered into a second amendment to sublease approximately 13,250 square feet of the Headquarters. This sublease expired in November 2024.

In February 2023, the Company entered into an agreement to sublease approximately 9,300 square feet of the Headquarters. This sublease will expire in July 2025. The sublessee pays its proportionate share of operating expenses for the space.

In August 2024, the Company approved a plan to transition operations from its laboratory and office space in Newark, California to its South San Francisco headquarters. The Company's intention is to sublease the Newark facility. The Company recorded an impairment charge of $2.2 million in general and administrative expenses to write-down the right-of-use asset and the leasehold improvements for the year ended December 31, 2024.

The components of lease expense were as follows:


				December 31,
	2024			2023			2022
	(In thousands)
Operating lease cost	$	6,494		$	6,626		$	6,638
Variable lease cost		3,410			3,149			2,785
Sublease income and reimbursement of variable lease cost		(2,444	)		(2,482	)		(1,872	)
Total	$	7,460		$	7,293		$	7,551

As of December 31, 2024, the weighted-average remaining lease term for operating leases was 4.2 years and the weighted-average discount rate was 8.6%. Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2024, 2023, and 2022 was $8.7 million, $8.4 million, and $8.2 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, 2024:


	(In thousands)
2025	$	9,161
2026		9,477
2027		9,804
2028		8,969
2029		2,209
Total undiscounted lease payments		39,620
Less: Present value adjustment		(6,490	)
Total lease liability	$	33,130

Net Loss Per Share - Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	19,235,060	18,348,694	16,925,023
Restricted Stock Units Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	8,365,063	6,092,101	3,044,618
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	10,723,681	12,096,237	13,713,272
Shares Committed Under 2019 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	146,316	160,356	167,133

Leases - Components of Lease Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Operating lease cost	$ 6,494	$ 6,626	$ 6,638
Variable lease cost	3,410	3,149	2,785
Sublease income and reimbursement of variable lease cost	(2,444)	(2,482)	(1,872)
Total	$ 7,460	$ 7,293	$ 7,551

Leases - Schedule of Lease Payments Owed Under Company's Operating Leases (Detail) $ in Thousands	Dec. 31, 2024 USD ($)
Leases [Abstract]
2025	$ 9,161
2026	9,477
2027	9,804
2028	8,969
2029	2,209
Total undiscounted lease payments	39,620
Less: Present value adjustment	(6,490)
Total lease liability	$ 33,130

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2024	Feb. 21, 2025	Jun. 30, 2024
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	82-2933343
Entity Public Float			$ 311.2
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	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		99,085,888
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 2025 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the end of the registrant’s 2024 fiscal year ended December 31, 2024.
Auditor Name	Ernst & Young LLP
Auditor Location	San Mateo, California
Auditor Firm ID	42
Auditor Opinion [Text Block]	Opinion on the Financial Statements We have audited the accompanying consolidated balance sheets of Alector, Inc. (the Company) as of December 31, 2024 and 2023, the related consolidated statements of operations and comprehensive loss, stockholders’ equity and cash flows for each of the three years in the period ended December 31, 2024, and the related notes (collectively referred to as the “consolidated financial statements”). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company at December 31, 2024 and 2023, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2024, in conformity with U.S. generally accepted accounting principles. We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (PCAOB), the Company’s internal control over financial reporting as of December 31, 2024, based on criteria established in Internal Control-Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 framework), and our report dated February 26, 2025 expressed an unqualified opinion thereon.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2024

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,
	2024		2023
	(In thousands)
Cash and cash equivalents	$	33,021	$	74,555
Restricted cash		1,846		1,546
Total cash, cash equivalents, and restricted cash	$	34,867	$	76,101

Summary of Segment Profit or Loss Including Significant Segment Expenses

The table below is a summary of the segment profit or loss, including significant segment expenses (in millions):


	Year Ended December 31,
	2024			2023			2022
Collaboration revenue	$	100.6		$	97.1		$	133.6
Less:
Compensation and benefits		73.2			72.8			69.1
Lab expenses and outside research services		14.8			11.6			19.8
Clinical supply costs		22.5			26.4			29.4
Clinical trials		58.6			62.3			59.2
Support functions		32.2			31.2			36.6
Other segment expenses(a)		44.3			44.5			57.4
Total operating expenses		245.6			248.8			271.5
Loss from operations		(145.0	)		(151.7	)		(137.9	)
Other income, net		26.1			26.6			7.8
Segment and consolidated loss before income taxes	$	(118.9	)	$	(125.1	)	$	(130.1	)

(a) Other segment expenses include consultants & contractor expense, contra-R&D expense for GSK collaboration, depreciation expense, impairment loss, restructuring expense, and stock-based compensation expense.

Collaboration Agreements - Additional Information (Details) $ in Thousands			1 Months Ended							3 Months Ended	12 Months Ended
	Aug. 17, 2021 USD ($)	Jul. 01, 2021 USD ($)	Dec. 31, 2023 USD ($)	May 31, 2023 USD ($)	Mar. 31, 2023 USD ($)	Feb. 28, 2023 USD ($)	Jan. 31, 2022 USD ($)	Jan. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense											$ 185,940	$ 192,115	$ 210,418
GlaxoSmithKline plc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Effective date of agreement	Aug. 17, 2021
Upfront payments		$ 700,000									$ 700,000
Additional milestone payments to support the enrollment	$ 500,000						$ 200,000
Milestone payment received	$ 500,000						$ 200,000
Percentage Of Development Cost											0.60
Collaboration revenue											$ 54,100	66,600	62,900
Deferred revenue			$ 247,400								195,800	247,400
Research and development expense											23,300	21,800
Reduction in transaction price										$ 4,200
Transaction price											571,600
Contract with customer, refund liability				$ 128,400
Maximum funding				140,500
Cumulative adjustment to revenue				26,900						$ 4,900
GlaxoSmithKline plc \| Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payments related to initiation of certain clinical studies and regulatory approval											1,500,000
Transaction price		$ 700,000
GlaxoSmithKline plc \| Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price				$ 571,600
GlaxoSmithKline plc \| Change in Estimate [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, refund liability											$ 57,900
Alector
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage Of Development Cost											0.40
AbbVie Biotechnology Limited
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payments									$ 205,000		$ 205,000
Additional milestone payments to support the enrollment			12,500		$ 17,800			$ 200,000	5,000		17,800
Milestone payment received			12,500		$ 17,800			$ 200,000	$ 5,000		17,800
Collaboration revenue											46,400	30,500	$ 70,700
Deferred revenue			$ 46,400									$ 46,400
Payment for enrollment of additional patients											$ 12,500
AbbVie Biotechnology Limited \| Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payments to support the enrollment						$ 12,500
Milestone payment received						$ 12,500

Restructuring - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Nov. 25, 2024 Employees	Mar. 29, 2023 Employees	Mar. 28, 2023
Restructuring Cost and Reserve [Line Items]
Percentage Of Reduction In Workforce			17.00%		11.00%
Number Of Reduction In Workforce \| Employees			41	30
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Operating Expenses	Operating Expenses
Restructuring Charges	$ 3.9	$ 1.7
Restructuring Reserve	$ 3.9

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

Stock-based Compensation - Summary of Restricted Stock Awards and Restricted Stock Units (Details) - Restricted Stock Awards And Restricted Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Shares
Number of Shares, Unvested, beginning balance	6,092,101	3,044,618
Number of Shares, Granted	5,905,124	5,142,444
Number of Shares, Vested	(3,001,132)	(1,584,448)
Number of Shares, Forfeited	(631,030)	(510,513)
Number of Shares, Unvested, ending balance	8,365,063	6,092,101
Weighted Average Grant Date Fair Value per Share
Weighted Average Grant Date Fair Value per Share, Unvested, beginning balance	$ 7.78	$ 11.68
Weighted Average Grant Date Fair Value per Share, Granted	4.69	6.62
Weighted Average Grant Date Fair Value per Share, Vested	7.48	10.41
Weighted Average Grant Date Fair Value per Share, Forfeited	7.06	10.03
Weighted Average Grant Date Fair Value per Share, Unvested, ending balance	$ 5.76	$ 7.78

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2024

Income Tax Disclosure [Abstract]

Schedule of Federal and State Income Tax Provision

The federal and state income tax provision for the year ended December 31, 2024, 2023 and 2022 are summarized as follows (in thousands):


	Year Ended December 31,
	2024		2023		2022
Current:
Federal	$	—	$	4,178	$	2,209
State		128		1,034		1,045
Income tax provision	$	128	$	5,212	$	3,254

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, 2024, 2023, and 2022 (in thousands):


	Year Ended December 31,
	2024			2023			2022
Tax benefit at federal statutory rate	$	(24,979	)	$	(26,188	)	$	(27,205	)
State income taxes		(1,680	)		(431	)		(344	)
Tax credits, net of uncertain tax positions		(8,182	)		(14,460	)		(15,138	)
Uncertain tax positions		179			612			2,687
Stock-based compensation		5,372			4,842			4,919
Change in valuation allowance		28,846			40,661			38,421
Other		572			176			(86	)
Income tax provision	$	128		$	5,212		$	3,254

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 and liabilities consist of (in thousands):


	December 31,
	2024			2023
Deferred tax assets:
Net operating loss	$	36,843		$	12,448
Accrued bonus		2,465			931
Tax credits		34,373			26,682
Stock-based compensation		15,456			15,311
Deferred revenue		46,085			63,004
Lease liability		7,796			9,030
Section 174 R&D capitalization		81,291			62,305
Refund liability		13,628			24,707
Other		1,619			521
Gross deferred tax assets		239,556			214,939
Less valuation allowance		(231,296	)		(204,550	)
Total deferred tax assets	$	8,260		$	10,389
Deferred tax liabilities:
Depreciation and amortization	$	(3,565	)	$	(4,544	)
Right-of-use assets		(4,695	)		(5,845	)
Gross deferred tax liabilities		(8,260	)		(10,389	)
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, 2024, 2023, and 2022 (in thousands):


Balance as of December 31, 2022	$	15,138
Decreases related to tax positions taken during the prior year		(594	)
Increase related to tax positions taken during the prior year	$	2,588
Increases related to tax positions taken during the current year		3,072
Balance as of December 31, 2023		20,204
Increase related to tax positions taken during the prior year		11
Increases related to tax positions taken during the current year		2,710
Balance as of December 31, 2024	$	22,925

Summary of Significant Accounting Policies - Summary of Segment Profit or Loss Including Significant Segment Expenses (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2024

Dec. 31, 2023

Dec. 31, 2022

Segment Reporting Information, Profit (Loss) [Abstract]

Revenues, Total

$ 100,600

$ 97,100

$ 133,600

Less:

Compensation and benefits

73,200

72,800

69,100

Lab Expenses and Outside Research Services

14,800

11,600

19,800

Clinical supply costs

22,500

26,400

29,400

Clinical Trials

58,600

62,300

59,200

Support functions

32,200

31,200

36,600

Other segment expenses

^[1]

44,300

44,500

57,400

Total operating expenses

245,555

248,802

271,451

Loss from operations

(145,000)

(151,700)

(137,900)

Other income, net

26,076

26,561

7,778

Segment and consolidated loss before income taxes

$ (118,900)

$ (125,100)

$ (130,100)

[1]	(a) Other segment expenses include consultants & contractor expense, contra-R&D expense for GSK collaboration, depreciation expense, impairment loss, restructuring expense, and stock-based compensation expense.

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net Income (Loss)	$ (119,049)	$ (130,391)	$ (133,310)

- Definition

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

+ References

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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jun. 30, 2018
Summary Of Significant Accounting Policies [Line Items]
Restricted cash	$ 1,846	$ 1,546		$ 1,500
Credit losses on marketable securities	0
Impairment loss on long-lived assets	2,205	0	$ 0
Income tax, Penalties and Interest accrued	$ 200	200
Defined contribution plan, maximum annual contributions per employee, percent	100.00%
Defined matching contribution plan	$ 1,100	$ 1,100	$ 1,200
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	srt:ChiefExecutiveOfficerMember
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description	The Company’s chief operating decision maker (CODM), its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. When evaluating the Company’s financial performance, the CODM reviews total revenues, total expenses and expenses by function.
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
Cash Pledged Agreement
Summary Of Significant Accounting Policies [Line Items]
Restricted cash	$ 300

Stock-based Compensation

12 Months Ended

Dec. 31, 2024

Share-Based Payment Arrangement [Abstract]

Stock-Based Compensation

8. Stock-based Compensation

The Company recognized stock-based compensation as follows (in thousands):


	Year Ended December 31,
	2024		2023		2022
Research and development	$	19,836	$	22,407	$	24,895
General and administrative		19,627		20,391		21,251
Total stock-based compensation	$	39,463	$	42,798	$	46,146

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,
	2024	2023	2022
Expected term (in years)	6.0	5.3 – 6.0	5.3 – 6.1
Expected volatility	81%	79% – 80%	79% – 80%
Risk free interest rate	4.3%	3.5% – 4.6%	1.5% – 4.3%
Dividend yield	—	—	—

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 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. Those companies were 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 in the foreseeable future.

2019 Equity Incentive Plan and 2022 Inducement 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 outstanding 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. Under the 2019 Plan, if an individual owns stock representing 10% or more of the outstanding shares, then for the individual’s incentive stock options, 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 stock options generally vest over a four-year period with one forty-eighth of the shares vesting each month or over a four-year period with 25% vesting at the one-year cliff and monthly thereafter. The RSUs generally vest over a period of three years with one-twelfth of the shares vesting quarterly.

On January 1, 2024, 4,243,984 shares were automatically added to the shares reserved for issuance under the 2019 Plan in accordance with the terms of the 2019 Plan. As of December 31, 2024, the Company had reserved 24,364,669 shares of common stock under the 2019 Plan, of which 6,781,827 shares were available for issuance of future awards.

On January 1, 2022, the Company adopted the 2022 Inducement Plan (Inducement Plan) and reserved 1,630,000 shares for issuance under the Inducement Plan for the grant of equity-based awards to individuals who were not previously employees or non-employee directors of the Company. On September 22, 2022, the Company increased the number of shares available for issuance under the 2022 Inducement Plan to a total of 3,300,000 shares. As of December 31, 2024, 1,547,349 shares were available for issuance of future awards under the Inducement Plan.

Option activity is 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, 2022		13,713,272		$	14.62
Granted		572,538			6.98
Exercised		(132,192	)		8.16
Forfeited		(2,057,381	)		14.21
Outstanding as of December 31, 2023		12,096,237			14.40
Granted		38,840			6.20
Exercised		—			—
Forfeited		(1,411,396	)		15.41
Outstanding as of December 31, 2024		10,723,681		$	14.24		5.7	$	—
Exercisable as of December 31, 2024		9,085,867		$	14.50		5.4	$	—
Vested and expected to vest as of December 31, 2024		10,723,681		$	14.24		5.7	$	—

For each in-the-money stock option, the aggregate intrinsic value is calculated as the number of shares underlying the stock option multiplied by the difference between the per share exercise price of the stock option and the fair market value of the Company’s common stock on the relevant date. The aggregate intrinsic value of options exercised was zero, less than $0.1 million, and $1.2 million for the years ended December 31, 2024, 2023, and 2022, respectively. The weighted-average grant-date fair value per share of options granted was $4.45, $4.93, and $8.42, for the years ended December 31, 2024, 2023, and 2022, respectively. As of December 31, 2024, total unrecognized stock-based compensation related to unvested stock options was $13.8 million, which the Company expects to recognize over a remaining weighted-average period of 1.5 years.

Restricted Stock Unit Activity

Activity for the RSUs is shown below. In May 2021 and January 2022, the Company issued RSUs with market conditions to certain executives, which are also included in the table below. The RSUs with market conditions are earned based on stock price performance and continued service by the employee. The RSUs with market conditions trigger vesting upon the Company’s stock price attaining a specified level over a specified period of time. The shares then vest quarterly over one year after attainment. The Company used a Monte Carlo simulation model to determine the fair value of the awards at the grant date. The Monte Carlo model uses the fair value inputs on the grant date to run simulations and take an average of possible outcomes. The total grant date fair value of the RSUs with market condition was $6.6 million to be amortized over an estimated weighted average service period of 2.1 years. Compensation expense related to awards with market-based conditions is recognized regardless of whether the market condition is ultimately satisfied if the related service has been provided.


	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock units as of December 31, 2022		3,044,618		$	11.68
Granted		5,142,444			6.62
Vested		(1,584,448	)		10.41
Forfeited		(510,513	)		10.03
Unvested restricted stock units as of December 31, 2023		6,092,101			7.78
Granted		5,905,124			4.69
Vested		(3,001,132	)		7.48
Forfeited		(631,030	)		7.06
Unvested restricted stock units as of December 31, 2024		8,365,063		$	5.76

As of December 31, 2024, total unrecognized stock-based compensation related to unvested RSUs issued to employees was $42.2 million, which the Company expects to recognize over a remaining weighted-average period of 2.2 years.

2019 Employee Stock Purchase Plan

The 2019 Employee Stock Purchase Plan (2019 ESPP) enables eligible employees of the Company to purchase shares of common stock at a discount. Each offering period is approximately six months long beginning on the first trading day on or after June 1 and December 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. On January 1, 2024, 591,397 shares were added to the shares reserved for issuance under the 2019 ESPP pursuant to the annual automatic increase. As of December 31, 2024, there was $0.1 million in unrecognized compensation expense related to the 2019 ESPP expected to be recognized over five months.

Income Taxes

12 Months Ended

Dec. 31, 2024

Income Tax Disclosure [Abstract]

Income Taxes

9. Income Taxes

The federal and state income tax provision for the year ended December 31, 2024, 2023 and 2022 are summarized as follows (in thousands):


	Year Ended December 31,
	2024		2023		2022
Current:
Federal	$	—	$	4,178	$	2,209
State		128		1,034		1,045
Income tax provision	$	128	$	5,212	$	3,254

A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2024, 2023, and 2022 (in thousands):


	Year Ended December 31,
	2024			2023			2022
Tax benefit at federal statutory rate	$	(24,979	)	$	(26,188	)	$	(27,205	)
State income taxes		(1,680	)		(431	)		(344	)
Tax credits, net of uncertain tax positions		(8,182	)		(14,460	)		(15,138	)
Uncertain tax positions		179			612			2,687
Stock-based compensation		5,372			4,842			4,919
Change in valuation allowance		28,846			40,661			38,421
Other		572			176			(86	)
Income tax provision	$	128		$	5,212		$	3,254

The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets and liabilities consist of (in thousands):


	December 31,
	2024			2023
Deferred tax assets:
Net operating loss	$	36,843		$	12,448
Accrued bonus		2,465			931
Tax credits		34,373			26,682
Stock-based compensation		15,456			15,311
Deferred revenue		46,085			63,004
Lease liability		7,796			9,030
Section 174 R&D capitalization		81,291			62,305
Refund liability		13,628			24,707
Other		1,619			521
Gross deferred tax assets		239,556			214,939
Less valuation allowance		(231,296	)		(204,550	)
Total deferred tax assets	$	8,260		$	10,389
Deferred tax liabilities:
Depreciation and amortization	$	(3,565	)	$	(4,544	)
Right-of-use assets		(4,695	)		(5,845	)
Gross deferred tax liabilities		(8,260	)		(10,389	)
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, 2024, 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 $26.7 million during the year ended December 31, 2024.

As of December 31, 2024, the Company had federal and state net operating loss (NOL) carryforwards of approximately $109.2 million and $223.5 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, 2024, the Company also had federal and California tax credit carryforward of approximately $34.9 million and $22.4 million, respectively. The federal tax credits will begin to expire in 2041 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, 2024, 2023, and 2022 (in thousands):


Balance as of December 31, 2022	$	15,138
Decreases related to tax positions taken during the prior year		(594	)
Increase related to tax positions taken during the prior year	$	2,588
Increases related to tax positions taken during the current year		3,072
Balance as of December 31, 2023		20,204
Increase related to tax positions taken during the prior year		11
Increases related to tax positions taken during the current year		2,710
Balance as of December 31, 2024	$	22,925

If the unrecognized tax benefits for uncertain tax positions as of December 31, 2024, 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 accrued interest and penalties of $0.2 million for the year ended December 31, 2024 and of $0.2 million for the year ended December 31, 2023. 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, 2024.

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 years ended December 31, 2024 and 2023, the Company recorded an uncertain tax position of $2.7 million and $5.1 million, respectively. The income tax provision for the years ended December 31, 2024 included changes to reserves related to prior year uncertain tax provisions of an increase of less than $0.1 million and decrease of zero. The income tax provision for the years ended December 31, 2023, included changes to reserves related to prior year uncertain tax provisions of an increase of $2.6 million and decrease of $0.6 million. 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.

- Definition

The entire disclosure for income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 740
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-Name Accounting Standards Codification
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-URI https://asc.fasb.org/1943274/2147482663/740-10-55-231

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Name Accounting Standards Codification
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12C

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12B

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Paragraph 2
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-URI https://asc.fasb.org/1943274/2147479360/740-10-S99-2

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482603/740-30-50-2

+ Details

Restructuring

12 Months Ended

Dec. 31, 2024

Restructuring and Related Activities [Abstract]

Restructuring

12. Restructuring

On March 28, 2023, the Company committed to a plan to reduce its workforce by approximately 11% to better align the Company’s resources with its previously announced strategic prioritization of its late-stage progranulin and TREM2 immuno-neurology programs. The Company initiated a reduction in force impacting approximately 30 employees across the organization effective March 29, 2023. For the year ended December 31, 2023, the Company incurred restructuring costs of approximately $1.7 million, primarily consisting of one-time charges related to the reduction in force, including personnel expenses such as salaries, one-time severance payments, and other benefits, which were included in operating expenses. All cash payments related to these expenses were paid out by early 2024.

On November 25, 2024, the Company committed to a plan to reduce its workforce by approximately 17% in order to align resources with the Company's strategic priorities. Based upon the results of the Company's INVOKE-2 Phase 2 clinical trial evaluating the safety and efficacy of AL002 in early Alzheimer’s disease, the Company is stopping the long term extension of the INVOKE-2 study. The Company initiated a reduction in force impacting approximately 41 employees across the organization. For the year ended December 31, 2024, the Company expects to incur restructuring costs of approximately $3.9 million, primarily consisting of one-time charges related to the reduction in force, including personnel expenses such as salaries, one-time severance payments, and other benefits, which were included in operating expenses. Cash payments related to these expenses will be paid out and the reduction in force is expected to be completed during the first half of 2025. Accrued liabilities associated with restructuring costs were $3.9 million as of December 31, 2024.

Collaboration Agreements

12 Months Ended

Dec. 31, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaboration Agreements

5. Collaboration Agreements

GSK

On July 1, 2021, the Company entered into a Collaboration and License Agreement with Glaxo Wellcome UK Limited, a subsidiary of GlaxoSmithKline plc (GSK), pursuant to which the Company and GSK collaborate on the global development and commercialization of progranulin-elevating monoclonal antibodies, including latozinemab, and AL101 (GSK Agreement). The GSK Agreement became effective on August 17, 2021.

Under the terms of the GSK Agreement, the Company received $700 million in upfront payments, of which $500 million was received in August 2021 and $200 million was received in January 2022. In addition, based on the development and commercialization plan for latozinemab and AL101, the Company may be eligible to receive up to an additional $1.5 billion in clinical development, regulatory, and commercial launch-related milestone payments. In the United States, the Company and GSK will equally share profits and losses from commercialization of latozinemab and AL101. Outside of the United States, the Company will be eligible for double-digit tiered royalties.

The Company and GSK will jointly develop latozinemab and AL101, with GSK conducting Phase 3 clinical trials for Alzheimer’s disease, Parkinson’s disease and other non-orphan indications. GSK will also conduct the initial Phase 2 trial for AL101 in Alzheimer’s disease. Development costs will be shared 60% by GSK and 40% by the Company, except that subject to the GSK Amendment (defined below), the Company will solely bear the development costs of the initial Phase 2 clinical trials under the development plan, and the parties will share manufacturing development costs equally.

In May 2023, the Company and GSK amended the GSK Agreement (GSK Amendment). Under the terms of the GSK Amendment, the Company is responsible for funding and sharing in GSK’s and the Company’s development costs up to $140.5 million for the conduct of the initial Phase 2 trial for AL101 in Alzheimer’s disease. The Company assessed the GSK Amendment in accordance with ASC 606 and concluded that the GSK Amendment was a contract modification to the GSK Agreement. Accordingly, the transaction price as of May 2023, was updated from $700 million to $571.6 million and the difference of $128.4 million was recorded as refund liability to collaboration partner for the expected cost reimbursement to GSK. The refund liability is an estimate of variable consideration calculated as the difference between the Company’s maximum funding of $140.5 million and the Company’s cost budget estimated using the expected value method. The Company determined that the modified performance obligation for the AL101 Alzheimer’s

disease program is performing development activities to support the initial Phase 2 trial, including license rights and know-how. The Company updated the cost-based input measure of progress for the modified performance obligation and recorded a cumulative catch-up adjustment to revenue of $26.9 million on the modification date relating to the performance obligation which was satisfied in prior periods.

During the three months ended September 30, 2023, as a result of the planned closure of the latozinemab Phase 2 trial and concurrent agreement by the Company to cost-share additional R&D, the Company determined there was a modification of the GSK Agreement, resulting in a decrease of the scope of the performance obligation associated with the latozinemab FTD-C9orf72 Phase 2 trial and an increase in the amount of R&D cost-shared by the Company in future periods. The impact of this additional cost share was accounted for as a refund liability, which reduced the transaction price for the GSK Agreement by $4.2 million. The refund liability is an estimate of variable consideration. The Company determined that the modified performance obligation for the latozinemab FTD-C9orf72 indication is performing the first Phase 2 development activities, including license rights and know-how. The Company updated the cost-based input measure of progress for the modified performance obligation and recorded a cumulative catch-up adjustment to revenue of $4.9 million on the modification date relating to the performance obligation that was satisfied in prior periods.

The Company concluded that the GSK Agreement is within the scope of ASC 808, Collaborative Arrangements, as both parties are active participants in the activities and are exposed to significant risks and rewards dependent on the success of the commercialization of latozinemab and AL101. Certain elements are required to be accounted for under ASC 606, Revenue From Contracts With Customers, where the counterparty is a customer for a good or service that is a distinct unit of account.

The Company determined that the distinct performance obligations under ASC 606 consisted of: (i) license and know-how to latozinemab FTD-GRN, which is currently in Phase 3 clinical development and (ii) the research and development activities, including license rights and know-how, relating to products in Phase 2 or earlier stages of development.

The transaction price at inception included fixed consideration consisting of the upfront payments of $700 million. The transaction price as of December 31, 2024 was decreased to $571.6 million due to the estimated refund liabilities created from the contract modifications. The Company reassessed the remaining estimated refund liabilities to collaboration partner as of December 31, 2024 to be $57.9 million. All potential future milestones and other payments were considered constrained at the inception of the GSK Agreement and as of December 31, 2024, since the Company could not conclude it was probable that a significant reversal in the amount of revenue recognized would not occur.

The respective standalone value for each of the performance obligations was allocated to the transaction price. The estimated SSP of each performance obligation was determined using discounted cash flows from the expected commercialization of latozinemab and AL101 and estimated research and development costs to be incurred by the Company in each of the initial Phase 2 clinical trials. The estimate of SSP for each performance obligation reflects management’s assumptions, which may include forecasted revenues, development timelines, discount rates, and probabilities of technical and regulatory success. For the license for FTD-GRN, the Company determined that GSK could benefit from the license at the time the license was granted and therefore, the related performance obligation was satisfied at a point in time. For the product candidates in Phase 2 or earlier stages of development, the Company determined that GSK could not benefit from the licenses without the corresponding development services that the Company has committed to perform due the earlier stage of development for these licenses. Except where agreed to otherwise, the Company will perform research and development activities through the end of the initial Phase 2 clinical trials. Revenue will be recognized over time as the research and development activities are performed. The Company will measure progress based on actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligations.

The research and development activities for products in Phase 3 clinical development were determined to be within the scope of ASC 808. Both parties will be active participants in the development, manufacturing, and commercialization of the product and are exposed to significant risks and rewards that are dependent on the commercial success of the products. The Company and GSK participate in profit and loss sharing for each program commensurate with each party’s cost-sharing responsibilities during research and development. ASC 808 does not provide recognition and measurement guidance. As such, the Company determined that ASC

730, Research and Development, was appropriate to analogize to based on the nature of the cost-sharing provision of the agreement. The Company has concluded that payments to or reimbursements from GSK related to these services will be accounted for as an increase to or reduction of research and development expenses, respectively.

Collaboration revenue under the GSK Agreement during the year ended December 31, 2024, 2023 and 2022, was $54.1 million, $66.6 million and $62.9 million, respectively, the entire amount of which was included in deferred revenue at the beginning of the period. The deferred revenue related to the GSK Agreement was $195.8 million and $247.4 million as of December 31, 2024 and 2023. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of initial Phase 2 clinical trials.

Costs associated with co-development activities performed under the agreement are included in research and development expenses in the consolidated statements of operations, with any reimbursement of costs by GSK reflected as a reduction of such expenses. For the year ended December 31, 2024 and 2023, the Company recognized a reduction of research and development expense of $23.3 million and $21.8 million, respectively, under the GSK Agreement.

AbbVie

The Company entered into 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 were received by the Company in October 2017 and January 2018, respectively. The Company was to perform research and development services for the two programs through the end of Phase 2 clinical trials, which were each considered to be separate performance obligations. AbbVie decided to terminate one of the two collaboration programs, the CD33 collaboration program, after AbbVie and Alector concluded that further development of AL003, the asset being developed under that program, was not warranted. AbbVie provided written notice to terminate the CD33 collaboration program on June 30, 2022. Accordingly, the Company is no longer developing that program and will not be eligible for any future milestones related to that program from AbbVie. The Company assessed its collaboration agreement with AbbVie in the context of the delivery of the research and development services.

In February 2023, the Company and AbbVie amended the AbbVie Agreement (AbbVie Amendment), which resulted in the Company receiving a $17.8 million milestone payment in March 2023 for the dosing of the first patient in an LTE trial. In addition, under the terms of the AbbVie Amendment, the Company was eligible to earn up to an additional $12.5 million to support the enrollment of additional patients to replace discontinuations, and the Company received the total $12.5 million in 2023. The performance obligations to conduct the LTE trial and enroll additional patients are not considered distinct from the AL002 program performance obligation.

In November 2024, the Company announced results from the INVOKE-2 Phase 2 clinical trial evaluating the safety and efficacy of AL002 in slowing disease progression in individuals with early AD. Treatment with AL002 resulted in sustained target engagement and pharmacodynamic responses indicative of microglial activation. However, AL002 failed to meet the primary endpoint of slowing of Alzheimer’s clinical progression as measured by the Clinical Dementia Rating Sum of Boxes (CDR®-SB), and there were no treatment effects that favored AL002 on secondary clinical and functional endpoints. Similarly, there were no significant effects on Alzheimer’s fluid biomarkers favoring AL002, and amyloid PET imaging demonstrated no treatment-related reduction of brain amyloid levels. Based upon the results, the Company is stopping the long term extension (LTE) study.

The transaction price as of December 31, 2024 included fixed consideration consisting of the upfront payments of $205.0 million, the $17.8 million LTE milestone payment, and the $12.5 million payment for enrollment of additional patients.

Collaboration revenue under the AbbVie Agreement during the years ended December 31, 2024 was $46.4 million, all of which was included in deferred revenue at the beginning of the period. Collaboration revenue under the AbbVie Agreement during the years ended December 31, 2023 and 2022 was $30.5 million and $70.7 million, respectively. Collaboration revenue under the AbbVie Agreement was fully recognized as of

December 31, 2024. The deferred revenue related to the AbbVie Agreement was $46.4 million as of December 31, 2023. The performance obligation was satisfied in December 2024, and all remaining deferred revenue has been recognized. In January 2025, AbbVie decided to terminate the TREM2 collaboration program, under which AL002 was being developed, which resulted in termination of the AbbVie Agreement.

Income Taxes - Schedule of Federal and State Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current:
Federal	$ 0	$ 4,178	$ 2,209
State	128	1,034	1,045
Income tax provision	$ 128	$ 5,212	$ 3,254

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(9))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (a)
-SubTopic 20
-Topic 740
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482659/740-20-45-2

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-SubTopic 10
-Topic 235
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

+ Details

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Income Tax Contingency [Line Items]
Valuation allowance deferred tax asset change in amount	$ 26,700,000
Income tax, Penalties and Interest accrued	200,000	$ 200,000
Uncertain tax position	2,700,000	5,100,000
Increase on uncertain tax provisions related to prior year	11,000	2,588,000
Decrease on uncertain tax provisions related to prior year	0	$ 594,000
Maximum
Income Tax Contingency [Line Items]
Increase on uncertain tax provisions related to prior year	100,000
California Tax Authority
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount	22,400,000
Federal Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward, net	109,200,000
Tax Credit Carryforward, Amount	$ 34,900,000
Tax Credit Beginning Expiration Year	2041
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, net	$ 223,500,000
Operating loss carry forward beginning expiration year	2030

Commitment and Contingencies

12 Months Ended

Dec. 31, 2024

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

4. Commitments and Contingencies

Contingencies

From time to time, the Company may be involved in litigation related to claims that arise in the ordinary course of its business activities. The Company accrues for these matters when it is probable that future expenditures will be made and these expenditures can be reasonably estimated. As of December 31, 2024, the Company does not believe that any such matters, individually or in the aggregate, will have a material adverse effect on the Company’s financial position, results of operations or cash flows.

Indemnification

The Company enters into customary indemnification arrangements in the ordinary course of business with vendors, clinical trial sites and other parties. Pursuant to these arrangements, the Company indemnifies, holds harmless and agrees to reimburse the indemnified parties for certain losses suffered or incurred by the indemnified party. The term of these indemnification agreements is generally perpetual any time after the execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these arrangements is not determinable. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. As a result, the Company has not recorded a liability related to such indemnification agreements as of December 31, 2024 or 2023. As permitted under Delaware law, the Company has entered into indemnification agreements with its directors and officers that requires it to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by law. The Company also has directors’ and officers’ insurance.

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2024

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:


	December 31,
	2024			2023
	(In thousands)
Computer equipment	$	2,407		$	2,372
Furniture and fixtures		2,439			2,430
Lab equipment		18,884			18,493
Leasehold improvements		26,357			26,616
Property and equipment, gross		50,087			49,911
Less accumulated depreciation and amortization		(32,942	)		(28,050	)
Total property and equipment, net	$	17,145		$	21,861

Summary of Accrued Liabilities

Accrued liabilities consist of the following:


	December 31,
	2024		2023
	(In thousands)
Accrued employee compensation	$	18,213	$	15,297
Accrued research and development costs		8,977		13,539
Accrued professional services		1,218		1,119
Other		482		423
Total accrued liabilities	$	28,890	$	30,378

Leases (Tables)

12 Months Ended

Dec. 31, 2024

Leases [Abstract]

Components of Lease Expense

The components of lease expense were as follows:


				December 31,
	2024			2023			2022
	(In thousands)
Operating lease cost	$	6,494		$	6,626		$	6,638
Variable lease cost		3,410			3,149			2,785
Sublease income and reimbursement of variable lease cost		(2,444	)		(2,482	)		(1,872	)
Total	$	7,460		$	7,293		$	7,551

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, 2024:


	(In thousands)
2025	$	9,161
2026		9,477
2027		9,804
2028		8,969
2029		2,209
Total undiscounted lease payments		39,620
Less: Present value adjustment		(6,490	)
Total lease liability	$	33,130

Leases - Additional Information (Detail) $ in Thousands		1 Months Ended			12 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Jun. 30, 2018 USD ($) ft²	Feb. 28, 2023 ft²	Nov. 30, 2021 ft²	Oct. 31, 2020 USD ($) ft²	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2019 USD ($)	Oct. 31, 2023 ft²	May 31, 2022 ft²
Lessee Lease Description [Line Items]
Tenant improvement allowance								$ 15,700
Amount from letter of credit	$ 1,500				$ 1,846	$ 1,546
Lease expiration year					2029
Operating lease, renewal period					10 years
Operating lease term					The lease expires in 2029 with an option to renew for a period of ten years.
Impairment charges					$ 2,205	0	$ 0
Weighted average remaining lease term for operating leases					4 years 2 months 12 days
Weighted average discount rate for operating leases					8.60%
Cash paid for amounts included In measurement of lease liabilities					$ 8,700	$ 8,400	$ 8,200
General and Administrative Expense
Lessee Lease Description [Line Items]
Impairment charges					$ 2,200
Offices and Laboratory Facilities \| South San Francisco
Lessee Lease Description [Line Items]
Area under lease \| ft²	105,000
Area under sublease \| ft²		9,300	7,100
Sublease expiration month and year		2025-07					2022-12
Additional area sublease expiration month and year						2023-11
Additional area of property in lease \| ft²									13,250	13,150
Offices and Laboratory Facilities \| South San Francisco
Lessee Lease Description [Line Items]
Tenant improvement allowance				$ 400
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. 06, 2028
Extend for an additional lease term				5 years

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Accrued Liabilities, Current [Abstract]
Accrued employee compensation	$ 18,213	$ 15,297
Accrued research and development costs	8,977	13,539
Accrued professional services	1,218	1,119
Other	482	423
Total accrued liabilities	$ 28,890	$ 30,378

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 33,021	$ 74,555
Marketable securities	380,376	474,306
Prepaid expenses and other current assets	11,420	16,946
Total current assets	424,817	565,807
Property and equipment, net	17,145	21,861
Operating lease right-of-use assets	19,951	25,195
Restricted cash	1,846	1,546
Other assets	4,544	7,418
Total assets	468,303	621,827
Current liabilities:
Accounts payable	2,215	3,775
Accrued clinical supply costs	6,989	5,215
Accrued liabilities	28,890	30,378
Deferred revenue, current portion	23,663	82,975
Payable to collaboration partner	5,914	7,703
Refund liability to collaboration partner, current portion	48,634	39,440
Operating lease liabilities, current portion	8,754	8,462
Total current liabilities	125,059	177,948
Deferred revenue, long-term portion	172,169	210,845
Long-term debt	9,389	0
Refund liability to collaboration partner, long-term portion	9,276	67,047
Operating lease liabilities, long-term portion	24,376	30,456
Other long-term liabilities	1,234	1,373
Total liabilities	341,503	487,669
Commitments and contingencies (Note 4)
Stockholders' equity:
Common stock, $0.0001 par value; 200,000,000 shares authorized; 99,085,888 and 84,879,693 shares issued and outstanding as of December 31, 2024 and December 31, 2023, respectively	9	8
Additional paid-in capital	955,657	844,044
Accumulated other comprehensive income	261	184
Accumulated deficit	(829,127)	(710,078)
Total stockholders' equity	126,800	134,158
Total liabilities and stockholders' equity	$ 468,303	$ 621,827

Debt - Additional Information (Details) - USD ($)		12 Months Ended
Debt - Additional Information (Details) - USD ($)	Nov. 14, 2024	Dec. 31, 2024
Debt Instrument [Line Items]
Maturity date		Dec. 01, 2028
Payment terms		At the Company’s option, the Company may prepay all or a portion of the outstanding Term Loans, subject to a prepayment premium equal to (a) 2.0% of the Term Loans being prepaid if the prepayment occurs during the 12 months following the Closing Date; (b) 1.5% of the Term Loans being prepaid if the prepayment occurs after the 12 month anniversary of the Closing Date but on or prior to the 24 month anniversary of the Closing Date; and (c) 0.5% of the Term Loans being prepaid if the prepayment occurs after 24 months following the Closing Date and prior to the Maturity Date. In addition, the Company will pay an end of term charge of (i) 2.45% if the Term Loans are prepaid or repaid within the first 24 months of the Closing Date; or (ii) 4.75% if the Term Loans are prepaid or repaid after 24 months from the Closing Date (including on the Maturity Date).
Facility charge	$ 250,000
Loan Agreement
Debt Instrument [Line Items]
Line of credit facility increase in rate of interest	5.00%
Proceeds from issuance of debt	$ 9,600,000
Debt issuance costs	400,000
Tranche One \| Loan Agreement
Debt Instrument [Line Items]
Amount drawn by the company	$ 10,000,000
Term Loan \| Loan Agreement
Debt Instrument [Line Items]
Face value of term loan		$ 10,000,000
Unamortized discount and unaccreted value of the end of term charge		$ 600,000
Imputed interest rate		12.20%
Fair value of term loan		$ 9,400,000
Term Loan \| Tranche Two \| Loan Agreement
Debt Instrument [Line Items]
Facility charge percentage of borrowing	1.00%
Secured Debt \| Term Loan \| Loan Agreement
Debt Instrument [Line Items]
Amount of term loan facility	$ 50,000,000
Interest rate		8.05%
Secured Debt \| Term Loan \| Prime Rate \| Loan Agreement
Debt Instrument [Line Items]
Interest rate		1.05%
Secured Debt \| Term Loan \| Tranche One \| Loan Agreement
Debt Instrument [Line Items]
Amount of term loan facility		$ 25,000,000
Secured Debt \| Term Loan \| Tranche Two \| Loan Agreement
Debt Instrument [Line Items]
Amount of term loan facility		$ 25,000,000

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2024

Share-Based Payment Arrangement [Abstract]

Summary of Recognized Stock-Based Compensation

The Company recognized stock-based compensation as follows (in thousands):


	Year Ended December 31,
	2024		2023		2022
Research and development	$	19,836	$	22,407	$	24,895
General and administrative		19,627		20,391		21,251
Total stock-based compensation	$	39,463	$	42,798	$	46,146

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,
	2024	2023	2022
Expected term (in years)	6.0	5.3 – 6.0	5.3 – 6.1
Expected volatility	81%	79% – 80%	79% – 80%
Risk free interest rate	4.3%	3.5% – 4.6%	1.5% – 4.3%
Dividend yield	—	—	—

Summary of Options to Purchase Common Stock

Option activity is 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, 2022		13,713,272		$	14.62
Granted		572,538			6.98
Exercised		(132,192	)		8.16
Forfeited		(2,057,381	)		14.21
Outstanding as of December 31, 2023		12,096,237			14.40
Granted		38,840			6.20
Exercised		—			—
Forfeited		(1,411,396	)		15.41
Outstanding as of December 31, 2024		10,723,681		$	14.24		5.7	$	—
Exercisable as of December 31, 2024		9,085,867		$	14.50		5.4	$	—
Vested and expected to vest as of December 31, 2024		10,723,681		$	14.24		5.7	$	—

Summary of Restricted Stock Awards and Restricted Stock Units

Activity for the RSUs is shown below.


	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock units as of December 31, 2022		3,044,618		$	11.68
Granted		5,142,444			6.62
Vested		(1,584,448	)		10.41
Forfeited		(510,513	)		10.03
Unvested restricted stock units as of December 31, 2023		6,092,101			7.78
Granted		5,905,124			4.69
Vested		(3,001,132	)		7.48
Forfeited		(631,030	)		7.06
Unvested restricted stock units as of December 31, 2024		8,365,063		$	5.76

Balance Sheet Components

12 Months Ended

Dec. 31, 2024

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components

6. Balance Sheet Components

Property and Equipment, Net

Property and equipment, net consists of the following:


	December 31,
	2024			2023
	(In thousands)
Computer equipment	$	2,407		$	2,372
Furniture and fixtures		2,439			2,430
Lab equipment		18,884			18,493
Leasehold improvements		26,357			26,616
Property and equipment, gross		50,087			49,911
Less accumulated depreciation and amortization		(32,942	)		(28,050	)
Total property and equipment, net	$	17,145		$	21,861

Accrued Liabilities

Accrued liabilities consist of the following:


	December 31,
	2024		2023
	(In thousands)
Accrued employee compensation	$	18,213	$	15,297
Accrued research and development costs		8,977		13,539
Accrued professional services		1,218		1,119
Other		482		423
Total accrued liabilities	$	28,890	$	30,378

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2024

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

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 financial institutions. 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 of $1.5 million relates to a letter of credit established for a lease entered into in June 2018. Restricted cash of $0.3 million was held as collateral to secure the use of corporate cards under a cash pledge agreement entered into in December 2024.


	Year Ended December 31,
	2024		2023
	(In thousands)
Cash and cash equivalents	$	33,021	$	74,555
Restricted cash		1,846		1,546
Total cash, cash equivalents, and restricted cash	$	34,867	$	76,101

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 or losses, 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 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, certificates of deposit, and commercial paper. 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 financial instruments include cash and cash equivalents, marketable securities, restricted cash, receivable from collaboration partner, current and noncurrent prepaid expenses, accounts payable, and accrued liabilities. The Company’s financial instruments approximate fair value due to their relatively short maturities.

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 determines whether an arrangement is or contains a lease at the inception of the arrangement. 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 its fair value. For the year ended December 31, 2024 the Company recognized an impairment loss of $2.2 million on the right-of-use asset and the leasehold improvements. The Company did not recognize an impairment loss on its long-lived assets for the years ended December 31, 2023 and 2022.

Revenue Recognition

The Company recognizes revenue when control of promised goods or services is 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. If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling price (SSP). The relative SSP for each performance obligation is estimated using externally sourced evidence if it is available. If externally sourced evidence is not available, we use our best estimate of the SSP for the performance obligation.

The Company recognizes collaboration revenue at a point in time if control of the promised good or service has been transferred to the customer. The Company recognizes collaboration revenue over time 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 re-evaluates the estimate of expected costs to satisfy the performance obligation each reporting period.

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 collaboration 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, CROs 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 is measured on the grant date based on the fair value of the awards. The fair value of options to purchase common stock is measured using the Black-Scholes option-pricing model. Stock-based compensation associated with restricted stock units (RSUs) that vest based only on a service condition is based on the fair value of the Company’s common stock on the grant date, which equals the closing price of the Company’s common stock on the grant date. The Company recognizes expense over the vesting period of the awards. Expense for options and RSUs that vest based only on a service condition is recognized on a straight-line basis.

The fair value of RSUs with market conditions is estimated using a Monte Carlo simulation model. The Monte Carlo model uses the fair value inputs on the grant date to run simulations and take an average of possible outcomes. Assumptions and estimates utilized in the model include the stock price on grant date, risk-free interest rate, dividend yield, expected stock volatility, and estimated period to achieve the market condition. The expense is recognized based on continued employment of the participants, regardless of achievement of the market condition. Expense related to the RSUs with market conditions is recognized using the accelerated attribution method.

The Company accounts for forfeitures as they occur for all awards.

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 been included in the financial statements. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial statement and tax bases of assets and liabilities and are measured using enacted tax rates and laws that will be in effect when the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date.

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 within the provision for taxes in the consolidated statements of operations. The Company accrued $0.2 million interest and penalties for the year ended December 31, 2024.

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 years ended December 31, 2024, 2023, and 2022, the Company made matching contributions of $1.1 million, $1.1 million, and $1.2 million, respectively.

Long-term debt

Long-term debt is initially recognized at cost and presented net of original issue discount and debt issuance costs on the consolidated balance sheets. Amortization of debt discount and debt issuance costs is recognized as interest expense over the period of the debt, using the effective interest rate method.

Segments

The Company operates in one segment. The Company’s chief operating decision maker (CODM), its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. When evaluating the Company’s financial performance, the CODM reviews total revenues, total expenses and expenses by function.

The table below is a summary of the segment profit or loss, including significant segment expenses (in millions):


	Year Ended December 31,
	2024			2023			2022
Collaboration revenue	$	100.6		$	97.1		$	133.6
Less:
Compensation and benefits		73.2			72.8			69.1
Lab expenses and outside research services		14.8			11.6			19.8
Clinical supply costs		22.5			26.4			29.4
Clinical trials		58.6			62.3			59.2
Support functions		32.2			31.2			36.6
Other segment expenses(a)		44.3			44.5			57.4
Total operating expenses		245.6			248.8			271.5
Loss from operations		(145.0	)		(151.7	)		(137.9	)
Other income, net		26.1			26.6			7.8
Segment and consolidated loss before income taxes	$	(118.9	)	$	(125.1	)	$	(130.1	)

Recent Accounting Pronouncements

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which expands disclosures in an entity’s income tax rate reconciliation table and disclosures regarding cash taxes paid both in the U.S. and foreign jurisdictions. The update will be effective for annual periods beginning after December 15, 2024. The Company is currently evaluating the impact that this guidance will have on the Company’s consolidated financial statements and disclosures.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2024

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Concentration of Credit Risk

Cash, Cash Equivalents, and Restricted Cash


	Year Ended December 31,
	2024		2023
	(In thousands)
Cash and cash equivalents	$	33,021	$	74,555
Restricted cash		1,846		1,546
Total cash, cash equivalents, and restricted cash	$	34,867	$	76,101

Marketable Securities

Fair Value of Financial Instruments

Level 1 – Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

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

Leases

Impairment of Long-Lived Assets

Revenue Recognition

Research and Development Costs

Stock-based Compensation

The Company accounts for forfeitures as they occur for all awards.

Comprehensive Loss

Income Taxes

Employee 401 (K) Plan

Employee 401(k) Plan

Long-term debt

Segments

The table below is a summary of the segment profit or loss, including significant segment expenses (in millions):


	Year Ended December 31,
	2024			2023			2022
Collaboration revenue	$	100.6		$	97.1		$	133.6
Less:
Compensation and benefits		73.2			72.8			69.1
Lab expenses and outside research services		14.8			11.6			19.8
Clinical supply costs		22.5			26.4			29.4
Clinical trials		58.6			62.3			59.2
Support functions		32.2			31.2			36.6
Other segment expenses(a)		44.3			44.5			57.4
Total operating expenses		245.6			248.8			271.5
Loss from operations		(145.0	)		(151.7	)		(137.9	)
Other income, net		26.1			26.6			7.8
Segment and consolidated loss before income taxes	$	(118.9	)	$	(125.1	)	$	(130.1	)

Recent Accounting Pronouncements

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2018
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 33,021	$ 74,555
Restricted cash	1,846	1,546			$ 1,500
Total cash, cash equivalents, and restricted cash	$ 34,867	$ 76,101	$ 155,795	$ 330,624

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ (24,979)	$ (26,188)	$ (27,205)
State income taxes	(1,680)	(431)	(344)
Tax credits, net of uncertain tax positions	(8,182)	(14,460)	(15,138)
Uncertain tax positions	179	612	2,687
Stock-based compensation	5,372	4,842	4,919
Change in valuation allowance	28,846	40,661	38,421
Other	572	176	(86)
Income tax provision	$ 128	$ 5,212	$ 3,254

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

+ Details

The Company and Liquidity - Additional Information (Details) - Over-Allotment Option [Member] $ / shares in Units, $ in Millions	Jan. 19, 2024 USD ($) $ / shares shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings \| shares	10,869,566
Share price \| $ / shares	$ 6.57
Aggregate net proceeds \| $	$ 71.1

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Dec. 31, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Collaboration revenue	$ 100,558	$ 97,062	$ 133,617
Operating expenses:
Research and development	185,940	192,115	210,418
General and administrative	59,615	56,687	61,033
Total operating expenses	245,555	248,802	271,451
Loss from operations	(144,997)	(151,740)	(137,834)
Other income, net	26,076	26,561	7,778
Loss before income taxes	(118,921)	(125,179)	(130,056)
Income tax expense	128	5,212	3,254
Net loss	(119,049)	(130,391)	(133,310)
Unrealized gain (loss) on marketable securities	77	4,759	(3,632)
Comprehensive loss	$ (118,972)	$ (125,632)	$ (136,942)
Net loss per share, basic	$ (1.23)	$ (1.56)	$ (1.62)
Net loss per share, diluted	$ (1.23)	$ (1.56)	$ (1.62)
Shares used in computing net loss per share, basic	96,588,177	83,733,730	82,467,587
Shares used in computing net loss per share, diluted	96,588,177	83,733,730	82,467,587

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 50,087	$ 49,911
Less accumulated depreciation and amortization	(32,942)	(28,050)
Total property and equipment, net	17,145	21,861
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,407	2,372
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,439	2,430
Lab equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	18,884	18,493
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 26,357	$ 26,616

Commitment and Contingencies - Additional Information (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023
Loss Contingencies [Line Items]
Indemnification agreement liability	$ 0	$ 0

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

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, 2024	Dec. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance	12,096,237	13,713,272
Number of Options, Granted	38,840	572,538
Number of Options, Exercised	0	(132,192)
Number of Options, Forfeited	(1,411,396)	(2,057,381)
Number of Options, Outstanding, ending balance	10,723,681	12,096,237
Number of Options, Exercisable	9,085,867
Number of Options, Vested and expected to vest	10,723,681
Weighted average exercise price, Outstanding, beginning balance	$ 14.4	$ 14.62
Weighted average exercise price, Granted	6.2	6.98
Weighted average exercise price, Exercised	0	8.16
Weighted average exercise price, Forfeited	15.41	14.21
Weighted average exercise price, Outstanding, ending balance	14.24	$ 14.4
Weighted average exercise price, Exercisable	14.5
Weighted average exercise price, Vested and expected to vest	$ 14.24
Weighted Average Remaining Contractual Term, Outstanding	5 years 8 months 12 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	5 years 4 months 24 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	5 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 0
Aggregate Intrinsic Value, Exercisable	0
Aggregate Intrinsic Value, Vested or expected to vest	$ 0

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, 2024	Dec. 31, 2023
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	$ 31,310	$ 67,101
Cash equivalents and marketable securities, Unrealized Gains	0	0
Cash equivalents and marketable securities, Unrealized Losses	0	0
Cash equivalents and marketable securities, Fair Market Value	31,310	67,101
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	72,360	178,232
Cash equivalents and marketable securities, Unrealized Gains	38	86
Cash equivalents and marketable securities, Unrealized Losses	(38)	(112)
Cash equivalents and marketable securities, Fair Market Value	72,360	178,206
Certificates of deposit \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	15,955	29,086
Cash equivalents and marketable securities, Unrealized Gains	4	63
Cash equivalents and marketable securities, Unrealized Losses	(1)	0
Cash equivalents and marketable securities, Fair Market Value	15,958	29,149
Commercial Paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	81,744	140,082
Cash equivalents and marketable securities, Unrealized Gains	51	85
Cash equivalents and marketable securities, Unrealized Losses	(15)	(34)
Cash equivalents and marketable securities, Fair Market Value	81,780	140,133
Corporate Bonds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	210,056	129,474
Cash equivalents and marketable securities, Unrealized Gains	301	173
Cash equivalents and marketable securities, Unrealized Losses	(79)	(78)
Cash equivalents and marketable securities, Fair Market Value	210,278	129,569
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	411,425	543,975
Cash equivalents and marketable securities, Unrealized Gains	394	407
Cash equivalents and marketable securities, Unrealized Losses	(133)	(224)
Cash equivalents and marketable securities, Fair Market Value	$ 411,686	$ 544,158

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturities of investments were less than one year	$ 364,800
Remaining contractual maturities, one year through two years	46,900
Sale of marketable securities	$ 2,568	$ 3,505	$ 0

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets:
Net operating loss	$ 36,843	$ 12,448
Accrued bonus	2,465	931
Tax credits	34,373	26,682
Stock-based compensation	15,456	15,311
Deferred revenue	46,085	63,004
Lease liability	7,796	9,030
Section 174 R&D capitalization	81,291	62,305
Refund liability	13,628	24,707
Other	1,619	521
Gross deferred tax assets	239,556	214,939
Less valuation allowance	(231,296)	(204,550)
Total deferred tax assets	8,260	10,389
Deferred tax liabilities:
Depreciation and amortization	(3,565)	(4,544)
Right-of-use assets	(4,695)	(5,845)
Gross deferred tax liabilities	(8,260)	(10,389)
Deferred tax assets, net	$ 0	$ 0