X0306 4 2021-05-07 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 2021-05-07 4 A 0 150000 0.00 A 1455226 D Common Stock 2612500 I See footnote Common Stock 712500 I See footnote Common Stock 712500 I See footnote Common Stock 712500 I See footnote The reported shares are represented by performance share units, or PSUs. Fifty percent of the award will be earned if the Issuer's average closing stock price is $30 per share for 40 consecutive trading days and an additional fifty percent will be earned if the average closing stock price is $40 per share for 40 consecutive trading days. Upon attainment of the performance metrics, the shares vest in four equal quarterly installments beginning on the first vesting date as set forth in the PSU award agreement. 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 2021-05-11