X0306 4 2021-07-02 0 0001653087 Alector, Inc. ALEC 0001757799 Yu Calvin C/O ALECTOR, INC. 131 OYSTER POINT BLVD, SUITE 600 SOUTH SAN FRANCISCO CA 94080 0 1 0 0 Vice President, Finance Common Stock 2021-07-02 4 M 0 18420 8.16 A 145106 D Common Stock 2021-07-02 4 M 0 11250 10.14 A 156356 D Common Stock 2021-07-02 4 M 0 31170 17.15 A 187526 D Common Stock 2021-07-02 4 S 0 90840 31.14 D 96686 D Common Stock 2021-07-02 4 S 0 20000 40.016 D 76686 D Common Stock 2021-07-06 4 M 0 1042 8.16 A 77728 D Common Stock 2021-07-06 4 M 0 625 10.14 A 78353 D Common Stock 2021-07-06 4 M 0 1667 17.15 A 80020 D Common Stock 2021-07-06 4 S 0 3334 34.65 D 76686 D Employee Stock Option (right to buy) 8.16 2021-07-02 4 M 0 18420 0.00 D 2028-07-02 Common Stock 18420 14118 D Employee Stock Option (right to buy) 10.14 2021-07-02 4 M 0 11250 0.00 D 2028-11-06 Common Stock 11250 10625 D Employee Stock Option (right to buy) 17.15 2021-07-02 4 M 0 31170 0.00 D 2029-11-06 Common Stock 31170 48830 D Employee Stock Option (right to buy) 8.16 2021-07-06 4 M 0 1042 0.00 D 2028-07-02 Common Stock 1042 13076 D Employee Stock Option (right to buy) 10.14 2021-07-06 4 M 0 625 0.00 D 2028-11-06 Common Stock 625 10000 D Employee Stock Option (right to buy) 17.15 2021-07-06 4 M 0 1667 0.00 D 2029-11-06 Common Stock 1667 47163 D The sales reported by the Reporting Person were effected pursuant to a Rule 10b5-1 trading plan adopted August 31, 2020. The sale price reported in column 4 of Table I represents the weighted average sale price of the shares purchased ranging from $40.00 to $40.32 per share. Upon request by the Commission staff, the Issuer, or a security holder of the Issuer, the Reporting Person will provide full information regarding the number of shares sold at each separate price. Shares subject to the option vest in 48 equal monthly installments beginning on August 2, 2018. One-fourth of the total number of shares subject to the option grant vested on November 1, 2019, and an additional 1/48th of the total number of shares vest monthly thereafter. Shares subject to the option vest in 48 equal monthly installments beginning on December 6, 2019. /s/ David Oh, by power of attorney 2021-07-07