SCHEDULE 13G/A 0000950170-24-012943 0001757770 XXXXXXXX LIVE 3 Common Stock, $0.0001 par value per share 12/31/2025 0001653087 Alector, Inc. 014442107 131 OYSTER POINT BLVD., SUITE 600 SOUTH SAN FRANCISCO CA 94080 Rule 13d-1(d) Arnon Rosenthal X1 4283550 3930375 4283550 3930375 8213925 N 7.4 IN With respect to Rows 5, 7, and 9, includes: (i) 1,736,295 shares of common stock held of record by Arnon Rosenthal (the "Reporting Person"), (ii) 132,804 shares of common stock that are issuable upon vesting of restricted stock units within 60 days of December 31, 2025 to the Reporting Person, and (iii) 2,414,451 shares of common stock that are issuable upon exercise of outstanding options within 60 days of December 31, 2025 to the Reporting Person. With respect to Rows 6, 8, and 9, includes (i) 1,972,875 shares of common stock held of record by The Rosenthal Family Revocable Trust Dated November 4, 1994, as restated on June 9, 1999, for which the Reporting Person serves as trustee; (ii) 652,500 shares of common stock held of record by The Adi Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee; (iii) 652,500 shares of common stock held of record by The Noam Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee and (iv) 652,500 shares of common stock held of record by The Shani Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee. With respect to Row 11, the percentage ownership is based on 109,151,472 shares of the Issuer's common stock outstanding as of October 31, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2025, filed with the Securities and Exchange Commission on November 6, 2025. Alector, Inc. 131 Oyster Point Blvd., Suite 600 South San Francisco, California 94080 Arnon Rosenthal c/o Alector, Inc. 131 Oyster Point Blvd., Suite 600 South San Francisco, California 94080 United States Y See Row 9 of cover page. See Row 5 of cover page for the Reporting Person. See Row 11 of cover page. See Row 6 of cover page for the Reporting Person. See Row 7 of cover page for the Reporting Person. See Row 8 of cover page for the Reporting Person. Y Y Y Y Y Y Arnon Rosenthal /s/Arnon Rosenthal 02/04/2026