X0206 3 2017-11-01 0 0001701108 Spero Therapeutics, Inc. SPRO 0001721017 Goyal Vikas C/O S.R. ONE, LIMITED 161 WASHINGTON STREET, SUITE 500 CONSHOHOCKEN PA 19428 1 0 0 0 Junior preferred stock Common Stock 244147 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 606024 I See Footnote The Junior preferred stock is convertible into the respective number of shares of common stock shown in column 3 in each case 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 Series A preferred stock is convertible into the respective number of shares of common stock shown in column 3 in each case 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 Series B preferred stock is convertible into the respective number of shares of common stock shown in column 3 in each case 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 Series C preferred stock is convertible into the respective number of shares of common stock shown in column 3 in each case 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. S.R. One, Limited, an indirect, wholly-owned subsidiary of GlaxoSmithKline plc, is the record holder of the shares reported herein. Vikas Goyal is a Principal at S.R. One, Limited and an employee of GlaxoSmithKline LLC, a wholly-owned subsidiary of GlaxoSmithKline plc. Mr. Goyal disclaims beneficial ownership of all the shares held by S.R. One, Limited and this report shall not be deemed an admission of beneficial ownership of such shares for the purposes of Section 16 or for any other purpose except to the extent of his pecuniary interest therein. Exhibit List Exhibit 24.1 - Power of Attorney /s/ Matthew Tikonoff, Attorney-in-Fact for Vikas Goyal 2017-11-01