X0306 4 2019-02-11 0 0001653087 Alector, Inc. ALEC 0001282930 GORDON CARL L C/O ORBIMED ADVISORS, LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 1 0 1 0 Common Stock 2019-02-11 4 C 0 405925 A 405925 I See footnotes Common Stock 2019-02-11 4 C 0 4282288 A 4688213 I See footnotes Common Stock 2019-02-11 4 C 0 2029626 A 6717839 I See footnotes Common Stock 2019-02-11 4 C 0 2280479 A 8998318 I See footnotes Common Stock 2019-02-11 4 C 0 1012282 A 10010600 I See footnotes Common Stock 2019-02-11 4 C 0 266437 A 10277037 I See footnotes Common Stock 2019-02-11 4 P 0 157939 19.00 A 10434976 I See footnotes Common Stock 2019-02-11 4 C 0 94075 A 94075 I See footnotes Common Stock 2019-02-11 4 C 0 992437 A 1086512 I See footnotes Common Stock 2019-02-11 4 C 0 470374 A 1556886 I See footnotes Common Stock 2019-02-11 4 C 0 528510 A 2085396 I See footnotes Common Stock 2019-02-11 4 C 0 234601 A 2319997 I See footnotes Common Stock 2019-02-11 4 C 0 85295 A 2405292 I See footnotes Common Stock 2019-02-11 4 P 0 50561 19.00 A 2455853 I See footnotes Common Stock 2019-02-11 4 P 0 203200 19.00 A 203200 I See footnotes Series A-1 Preferred Stock 2019-02-11 4 C 0 405925 0.00 D Common Stock 405925 0 I See footnotes Series A-1 Preferred Stock 2019-02-11 4 C 0 94075 0.00 D Common Stock 94075 0 I See footnotes Series A-2 Preferred Stock 2019-02-11 4 C 0 4282288 0.00 D Common Stock 4282288 0 I See footnotes Series A-2 Preferred Stock 2019-02-11 4 C 0 992437 0.00 D Common Stock 992437 0 I See footnotes Series B Preferred Stock 2019-02-11 4 C 0 2029626 0.00 D Common Stock 2029626 0 I See footnotes Series B Preferred Stock 2019-02-11 4 C 0 470374 0.00 D Common Stock 470374 0 I See footnotes Series C Preferred Stock 2019-02-11 4 C 0 2280479 0.00 D Common Stock 2280479 0 I See footnotes Series C Preferred Stock 2019-02-11 4 C 0 528510 0.00 D Common Stock 528510 0 I See footnotes Series D Preferred Stock 2019-02-11 4 C 0 1012282 0.00 D Common Stock 1012282 0 I See footnotes Series D Preferred Stock 2019-02-11 4 C 0 234601 0.00 D Common Stock 234601 0 I See footnotes Series E Preferred Stock 2019-02-11 4 C 0 266437 0.00 D Common Stock 266437 0 I See footnotes Series E Preferred Stock 2019-02-11 4 C 0 85295 0.00 D Common Stock 85295 0 I See footnotes 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 automatically converted into shares of Common Stock on a one to one basis immediately prior to the completion of the Issuer's initial public offering of Common Stock and has no expiration date. The Shares are held of record by OrbiMed Private Investments IV-AL, LP ("OrbiMed IV-AL"). OrbiMed Capital GP IV LLC ("OrbiMed GP"), is the general partner of OrbiMed IV-AL. OrbiMed Advisors LLC ("OrbiMed Advisors"), a registered adviser under the Investment Advisers Act of 1940, as amended, is the managing member of OrbiMed GP. By virtue of such relationships, OrbiMed GP and OrbiMed Advisors may be deemed to have voting and investment power over the securities held by OrbiMed IV-AL and as a result may be deemed to have beneficial ownership over such securities. The Reporting Person is a member of OrbiMed Advisors. Each of OrbiMed GP, OrbiMed Capital LLC ("OrbiMed Capital"), OrbiMed Advisors and the Reporting Person disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its or his pecuniary interest therein, if any. This report on Form 3 shall not be deemed an admission that any of the reporting persons is a beneficial owner of such securities for the purpose of Section 16 of the Exchange Act, or for any other purpose. The Shares are held of record by OrbiMed Private Investments IV-AL (Feeder), LP ("OrbiMed IV-AL (Feeder)"). OrbiMed GP is the general partner of OrbiMed IV-AL (Feeder). OrbiMed Advisors, a registered adviser under the Investment Advisers Act of 1940, as amended, is the managing member of OrbiMed GP. By virtue of such relationships, OrbiMed GP and OrbiMed Advisors may be deemed to have voting and investment power over the securities held by OrbiMed IV-AL (Feeder) and as a result may be deemed to have beneficial ownership over such securities. The Reporting Person is a member of OrbiMed Advisors. The Shares are held of record by OrbiMed Partners Master Fund Limited ("OPM"). OrbiMed Capital, a registered investment adviser under the Investment Advisers Act, acts as the investment advisor to OPM. By virtue of such relationships, OrbiMed Capital may be deemed to have voting and investment power over the securities held by OPM and as a result may be deemed to have beneficial ownership over such securities. /s/ Stephanie Yonker, attorney-in-fact 2019-02-11