X0206 3 2017-11-01 0 0001701108 Spero Therapeutics, Inc. SPRO 0001030575 FORMELA JEAN FRANCOIS C/O SPERO THERAPEUTICS, INC. 675 MASSACHUSETTS AVENUE, 14TH FLOOR CAMBRIDGE MA 02139 1 0 0 0 Common Stock 42781 I See Footnote Junior Preferred Stock Common Stock 245605 I See Footnote Series A Preferred Stock Common Stock 246272 I See Footnote Series B Preferred Stock Common Stock 328992 I See Footnote Series C Preferred Stock Common Stock 513318 I See Footnote Series C Preferred Stock Common Stock 602589 I See Footnote 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. Shares held 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. Dr. Formela is a member of AVA IX LLC and disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein, if any. 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. Shares held by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X") The general partner of Atlas Venture Fund X is Atlas Venture Associates X, L.P. ("AVA X LP"). Atlas Venture Associates X, LLC ("AVA X LLC") is the general partner of AVA X LP. Dr. Formela is a member of AVA X LLC and disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein, if any. Exhibit 24.1 - Power of Attorney /s/ Frank Castellucci, Attorney-in-Fact for Jean-Francois Formela 2017-11-01