X0306 4 2019-02-11 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 2019-02-11 4 C 0 500000 A 500000 I See footnote Common Stock 2019-02-11 4 C 0 5274725 A 5774725 I See footnotes Common Stock 2019-02-11 4 C 0 2500000 A 8274725 I See footnotes Common Stock 2019-02-11 4 C 0 2808989 A 11083714 I See footnotes Common Stock 2019-02-11 4 C 0 1246883 A 12330597 I See footnotes Common Stock 2019-02-11 4 C 0 527597 A 12858194 I See footnotes Series A-1 Preferred Stock 2019-02-11 4 C 0 500000 0.00 D Common Stock 500000 0 I See footnote Series A-2 Preferred Stock 2019-02-11 4 C 0 5274725 0.00 D Common Stock 5274725 0 I See footnote Series B Preferred Stock 2019-02-11 4 C 0 2500000 0.00 D Common Stock 2500000 0 I See footnote Series C Preferred Stock 2019-02-11 4 C 0 2808989 0.00 D Common Stock 2808989 0 I See footnote Series D Preferred Stock 2019-02-11 4 C 0 1246883 0.00 D Common Stock 1246883 0 I See footnote Series E Preferred Stock 2019-02-11 4 C 0 527597 0.00 D Common Stock 527597 0 I See footnote Each of the Series A-1 Preferred Stock, Series A-2 Preferred Stock, Series B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock and Series E Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. 27,606 shares are held by Polaris Venture Partners Founders' Fund VI, L.P ("PVPFF VI"), 379,174 shares are held by Polaris Venture Partners VI (AIV), L.P. ("PVP VI AIV") and 93,220 shares are held 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"). PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. 291,223 shares are held by PVPFF VI, 4,000,075 shares are held by PVP VI AIV and 983,427 shares are held by PVP VI Feeder. PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. 138,030 shares are held by PVPFF VI, 1,895,867 shares are held by PVP VI AIV and 466,103 shares are held by PVP VI Feeder. PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. 155,087 shares are held by PVPFF VI, 2,130,190 shares are held by PVP VI AIV and 523,712 shares are held by PVP VI Feeder. PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. 68,842 shares are held by PVPFF VI, 945,571 shares are held by PVP VI AIV and 232,470 shares are held by PVP VI Feeder. PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. 29,129 shares are held by PVPFF VI and 498,468 shares are held by PVP VI. PVM disclaims beneficial ownership of the securities held by the Polaris Funds, and this report shall not be deemed an admission that PVM is the beneficial owner of such securities, except to the extent of its pecuniary interest therein, if any. /s/ Terrance McGuire, Managing Member of Polaris Venture Management Co. VI, L.L.C. 2019-02-13 /s/ Terrance McGuire, Managing Member of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI (AIV), L.P. 2019-02-13 /s/ Terrance McGuire, Managing Member of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners Founders' Fund VI, L.P. 2019-02-13 /s/ Terrance McGuire, Managing Member of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI, L.P. 2019-02-13 /s/ Terrance McGuire, Managing Member of Polaris Venture Management Co. VI, L.L.C., general partner of PVP VI (AIV) Feeder Corp. Holding Partnership, L.P. 2019-02-13