X0206 3 2017-11-01 0 0001701108 Spero Therapeutics, Inc. SPRO 0001671219 Atlas Venture Fund IX, L.P. 25 FIRST STREET, SUITE 303 CAMBRIDGE MA 02141 0 0 1 0 0001671208 Atlas Venture Associates IX, L.P. 25 FIRST STREET, SUITE 303 CAMBRIDGE MA 02141 0 0 1 0 0001671207 Atlas Venture Associates IX, LLC 25 FIRST STREET, SUITE 303 CAMBRIDGE MA 02141 0 0 1 0 Common Stock 42781 D Junior Preferred Stock Common Stock 245605 D Series A Preferred Stock Common Stock 246272 D Series B Preferred Stock Common Stock 328992 D Series C Preferred Stock Common Stock 513318 D The number of shares reported reflects a 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017. The shares of Junior preferred stock are convertible into the number of shares of common stock shown in Column 3, calculated using the conversion ratio set forth in the Issuer's Amended and Restated Certificate of Incorporation, as amended, at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares have no expiration date. Share numbers give effect to the 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017. The shares of Series A preferred stock are convertible into the number of shares of common stock shown in Column 3, calculated using the conversion ratio set forth in the Issuer's Amended and Restated Certificate of Incorporation, as amended, at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares have no expiration date. The shares of Series B preferred stock are convertible into the number of shares of common stock shown in Column 3, calculated using the conversion ratio set forth in the Issuer's Amended and Restated Certificate of Incorporation, as amended, at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares have no expiration date. The shares of Series C preferred stock are convertible into the number of shares of common stock shown in Column 3, calculated using the conversion ratio set forth in the Issuer's Amended and Restated Certificate of Incorporation, as amended, at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares have no expiration date. The shares are held directly by Atlas Venture Fund IX, L.P. ("Atlas Venture Fund IX"). The general partner of Atlas Venture Fund IX is Atlas Venture Associates IX, L.P. ("AVA IX LP"). Atlas Venture Associates IX, LLC ("AVA IX LLC") is the general partner of AVA IX LP. Each of AVA IX LP and AVA IX LLC disclaims beneficial ownership of the shares held by Atlas Venture Fund IX, except to the extent of its pecuniary interest therein. Atlas Venture Fund IX, LP, By: Atlas Venture Associates IX, LP, its general partner, By: Atlas Venture Associates IX, LLC, its general partner, By: Frank Castellucci, General Counsel , /s/ Frank Castellucci 2017-11-01 Atlas Venture Associates IX, LP., By: Atlas Venture Associates IX, LLC, its general partner, By: Frank Castellucci, General Counsel, /s/ Frank Castellucci 2017-11-01 Atlas Venture Associates IX, LLC, By: Frank Castellucci, General Counsel, /s/ Frank Castellucci 2017-11-01