X0306 4 2021-09-22 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 2021-09-22 4 S 0 94717 25.289 D 2317783 I See footnote Common Stock 2021-09-22 4 S 0 5283 26.1007 D 2312500 I See footnote Common Stock 2021-09-23 4 S 0 9625 25.0979 D 2302875 I See footnote Common Stock 1455226 D Common Stock 712500 I See footnote Common Stock 712500 I See footnote Common Stock 712500 I See footnote The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.00 to $25.98, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (1), (3) and (4) to this Form 4. 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 price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $26.00 to $26.22, inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.00 to $25.26, inclusive. /s/ David Oh, by power of attorney 2021-09-23