As filed with the Securities and Exchange Commission on January 29, 2020.

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

Under

The Securities Act of 1933

ALECTOR, INC.

(Exact name of Registrant as specified in its charter)

131 Oyster Point Blvd. Suite 600

South San Francisco, California 94080

415-231-5660

(Address, including zip code, and telephone number, including area code, of Registrant’s principal executive offices)

Arnon Rosenthal, Ph.D.

Chief Executive Officer

Alector, Inc.

131 Oyster Point Blvd. Suite 600

South San Francisco, California 94080

415-231-5660

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this registration statement.

If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  ☐

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☒ 333-236094

If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☒

CALCULATION OF REGISTRATION FEE

This registration statement shall become effective upon filing with the Securities and Exchange Commission in accordance with Rule 462(b) of the Securities Act of 1933, as amended.

EXPLANATORY NOTE AND INCORPORATION BY REFERENCE

Alector, Inc., a Delaware corporation (“Alector”), is filing this registration statement with the Securities and Exchange Commission pursuant to Rule 462(b) under the Securities Act of 1933, as amended. This registration statement relates to the public offering of securities contemplated by the registration statement on Form S-1 (File No. 333-236094) originally filed on January 27, 2020, as amended, or the Prior Registration Statement, and which the Securities and Exchange Commission declared effective on January 29, 2020.

Alector is filing this registration statement for the sole purpose of increasing by 1,552,500 shares the number of shares of its common stock, par value $0.0001 per share, to be registered for sale. The additional shares that are being registered for sale are in an amount and at a price that together represent no more than 20% of the maximum aggregate offering price set forth in the Calculation of Registration Fee table contained in the Prior Registration Statement. The information set forth in the Prior Registration Statement, and all exhibits to the Prior Registration Statement, are hereby incorporated by reference into this registration statement.

The required opinions and consents are listed on the Exhibit Index attached hereto and are filed herewith.

EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on the 29th day of January, 2020.

Pursuant to the requirements of the Securities Act of 1933, as amended, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

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