X0306 4 2021-11-10 0 0001653087 Alector, Inc. ALEC 0001757770 Rosenthal Arnon C/O ALECTOR, INC. 131 OYSTER POINT BLVD, SUITE 600 SOUTH SAN FRANCISCO CA 94080 1 1 0 0 Chief Executive Officer Common Stock 2021-11-10 4 S 0 55000 25.00 D 2027875 I See footnote Common Stock 2021-11-11 4 S 0 55000 25.00 D 1972875 I See footnote Common Stock 652500 I See footnote Common Stock 652500 I See footnote Common Stock 652500 I See footnote Common Stock 1455226 D The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan. The reported securities are held directly by The Rosenthal Family Revocable Trust Dated November 4, 1994, as restated on June 9, 1999, for which the Reporting Person serves as trustee. The reported securities are held directly by the Adi Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee. The reported securities are held directly by the Noam Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee. The reported securities are held directly by the Shani Rosenthal 2007 Trust dated March 27, 2007, for which the Reporting Person serves as trustee. /s/ David Oh, by power of attorney 2021-11-12