Spero Therapeutics, Inc. — Director's Dealing 2017

Nov 1, 2017

SEC Form 3 — Initial Statement of Beneficial Ownership

Issuer: Spero Therapeutics, Inc. (SPRO)
CIK: 0001701108
Period of Report: 2017-11-01

Reporting Person: Atlas Venture Fund IX, L.P. (10% Owner)
Reporting Person: Atlas Venture Associates IX, L.P. (10% Owner)
Reporting Person: Atlas Venture Associates IX, LLC (10% Owner)

Holdings (Non-Derivative)

Security	Shares	Ownership
Common Stock	42781	Direct

Holdings (Derivative)

Security	Exercise Price	Expiration	Underlying	Shares	Ownership
Junior Preferred Stock	$		Common Stock (245605)		Direct
Series A Preferred Stock	$		Common Stock (246272)		Direct
Series B Preferred Stock	$		Common Stock (328992)		Direct
Series C Preferred Stock	$		Common Stock (513318)		Direct

Footnotes

F1: 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.

F2: 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.

F3: Share numbers give effect to the 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017.

F4: 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.

F5: 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.

F6: 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.

F7: 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.