X0306 4 2020-07-20 0 0001532346 Atreca, Inc. BCEL 0001263508 BAKER BROS. ADVISORS LP 860 WASHINGTON STREET 3RD FLOOR NEW YORK NY 10014 0 0 1 0 0001580575 Baker Bros. Advisors (GP) LLC 860 WASHINGTON STREET 3RD FLOOR NEW YORK NY 10014 0 0 1 0 0001087940 BAKER FELIX 860 WASHINGTON STREET 3RD FLOOR NEW YORK NY 10014 0 0 1 0 0001087939 BAKER JULIAN 860 WASHINGTON STREET, 3RD FLOOR NEW YORK NY 10014 0 0 1 0 Class B Common Stock 2020-07-20 4 P 0 52880 16.00 A Class A Common Stock 52880 542114 I See Footnotes Class B Common Stock 2020-07-20 4 P 0 728370 16.00 A Class A Common Stock 728370 6173327 I See Footnotes The Class B common stock ("Class B Common Stock") of Atreca, Inc. (the "Issuer") is non-voting and has no expiration date. Each share of Class B Common Stock is convertible into one share of Class A common stock ("Class A Common Stock") of the Issuer at any time at the option of the holder without consideration subject to a 4.99% beneficial ownership limitation which may be increased or decreased by the holder upon 61 days' notice to the Issuer. 667, L.P. ("667") and Baker Brothers Life Sciences, L.P. ("Life Sciences", and together with "667", the "Funds") purchased 52,880 and 728,370 shares of Class B Common Stock, respectively, for $16.00 per share in an underwritten public offering that closed on July 20, 2020. After giving effect to the transaction reported herein and as a result of their ownership interest in (i) Baker Biotech Capital, L.P. and (ii) 667, Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in Class B Common Stock reported in column 9 of Table II directly held by 667, a limited partnership of which the sole general partner is Baker Biotech Capital, L.P., a limited partnership of which the sole general partner is Baker Biotech Capital (GP), LLC, due to their interest in 667 and Baker Biotech Capital, L.P.'s right to receive an allocation of a portion of the profits from 667. Baker Bros. Advisors LP (the "Adviser") serves as the investment adviser to the Funds. In connection with the services provided by the Adviser, the Adviser receives an asset-based management fee that does not confer any pecuniary interest in the securities held by the Funds. Baker Bros. Advisors (GP) LLC (the "Adviser GP") is the Adviser's sole general partner. Julian C. Baker and Felix J. Baker are managing members of the Adviser GP. The Adviser has complete and unlimited discretion and authority with respect to the investment and voting power of the securities held by the Funds. The general partners of the Funds relinquished to the Adviser all discretion and authority with respect to the investment and voting power of the securities held by the Funds. After giving effect to the transaction reported herein and as a result of their ownership interest in (i) Baker Brothers Life Sciences Capital, L.P. and (ii) Life Sciences, Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in Class B Common Stock reported in column 9 of Table II directly held by Life Sciences, a limited partnership of which the sole general partner is Baker Brothers Life Sciences Capital, L.P., a limited partnership of which the sole general partner is Baker Brothers Life Sciences Capital (GP), LLC, due to their interest in Life Sciences and Baker Brothers Life Sciences Capital, L.P.'s right to receive an allocation of a portion of the profits from Life Sciences. Julian C. Baker, Felix J. Baker, the Adviser GP and the Adviser disclaim beneficial ownership of the securities held directly by the Funds except to the extent of their pecuniary interest therein, and this report shall not be deemed an admission that any of Julian C. Baker, Felix J. Baker, the Adviser GP or the Adviser is a beneficial owner of such securities for purposes of Section 16 or any other purpose. By: Baker Bros. Advisors LP, Name: Scott L. Lessing, Title: President /s/ Scott L. Lessing 2020-07-20 By: Baker Bros. Advisors (GP) LLC, Name: Scott L. Lessing, Title: President /s/ Scott L. Lessing 2020-07-20 /s/ Felix J. Baker 2020-07-20 /s/ Julian C. Baker 2020-07-20