X0306 4 2020-06-23 0 0001653087 Alector, Inc. ALEC 0001665594 Polaris Venture Management Co. VI, L.L.C. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 0001767127 Polaris Venture Partners VI (AIV), L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 0001575691 Polaris Venture Partners Founders' Fund VI, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 0001493463 Polaris Venture Partners VI, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 0001767130 PVP VI (AIV) Feeder Corp. Holding Partnership, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 Common Stock 2020-06-23 4 S 0 4377 31.5428 D 12853817 I See footnote 242 shares were sold by Polaris Venture Partners Founders' Fund VI, L.P ("PVPFF VI"), 3,153 shares were sold by Polaris Venture Partners VI (AIV), L.P. ("PVP VI AIV") and 982 shares were sold by PVP VI (AIV) Feeder Corp. Holding Partnership, L.P. ("PVP VI Feeder" and, together with PVP VI AIV, PVPFF VI, and Polaris Venture Partners VI, L.P. ("PVP VI"), the "Polaris Funds"). The general partner of each of the Polaris Funds is Polaris Venture Management Co. VI, L.L.C. ("PVM"). Each of Amir Nashat, Brian Chee, David Barrett, Bryce Youngren, Jonathan Flint, and Terrance McGuire, a member of the Issuer's board of directors, are managing members of PVM (collectively, the "Managing Members") and may be deemed to have shared power to vote and dispose of the shares held by the Polaris Funds. (Continued from footnote 1) Each of the Managing Members and PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interest therein. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $31.50 to $31.61, inclusive. The Reporting Persons undertake to provide to the Issuer, any security holder of the Issuer or the staff of the Securities Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (3) to this Form 4. 709,675 shares are held by PVPFF VI, 9,347,724 shares are held by PVP VI AIV, 2,297,950 shares are held by PVP VI Feeder and 498,468 shares are held by PVP VI. Each of the Managing Members and PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interest therein. /s/ Lauren Crockett, as attorney in fact for Polaris Venture Management Co. VI, L.L.C. 2020-06-25 /s/ Lauren Crockett, as attorney in fact for Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI (AIV), L.P. 2020-06-25 /s/ Lauren Crockett, as attorney in fact for Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners Founders' Fund VI, L.P. 2020-06-25 /s/ Lauren Crockett, as attorney in fact for Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI, L.P. 2020-06-25 /s/ Lauren Crockett, as attorney in fact for Polaris Venture Management Co. VI, L.L.C., general partner of PVP VI (AIV) Feeder Corp. Holding Partnership, L.P. 2020-06-25