X0306 4 2016-12-12 0 0001445283 PROTEOSTASIS THERAPEUTICS, INC. PTI 0001583977 Cormorant Asset Management, LLC 200 CLARENDON STREET, 52ND FLOOR BOSTON MA 02116 0 0 1 0 0001599214 Chen Bihua C/O CORMORANT ASSET MANAGEMENT, LLC 200 CLARENDON STREET, 52ND FLOOR BOSTON MA 02116 1 0 1 0 0001618451 Cormorant Global Healthcare GP, LLC 200 CLARENDON STREET, 52ND FLOOR BOSTON MA 02116 0 0 1 0 0001618442 Cormorant Global Healthcare Master Fund, LP 200 CLARENDON STREET, 52ND FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2016-12-12 4 P 0 100000 10.7895 A 3782726 I See Footnotes Common Stock 2016-12-13 4 P 0 21301 10.9833 A 3804027 I See Footnotes Shares reported herein are held by Cormorant Global Healthcare Master Fund, LP (the "Master Fund") and by a managed account (the "Account"). Cormorant Asset Management, LLC ("Cormorant") serves as the investment manager of the Master Fund and the Account. Cormorant Global Healthcare GP, LLC ("GP LLC") serves as General Partner of the Master Fund. Bihua Chen serves as manager of Cormorant and GP LLC. GP LLC may be deemed to beneficially own the shares held by the Master Fund, and Cormorant and Ms. Chen may be deemed to beneficially own the shares held by the Master Fund and the Account. Each of the Reporting Persons disclaims beneficial ownership of the shares reported herein except to the extent of its or her pecuniary interest therein, and the filing of this Form 4 shall not be construed as an admission that any of the Reporting Persons is the beneficial owner of any such shares for purposes of Section 16(a) of the Securities Exchange Act of 1934 or for any other purpose. Represents the weighted average purchase price of shares purchased in a series of open market transactions on the transaction date at prices ranging from $10.57 to $11.00 per share. The Reporting Persons undertake to provide, upon request by the staff of the Securities and Exchange Commission, the issuer, or a security holder of the issuer, full information regarding the number of shares purchased at each price. Of these shares, 83,150 were purchased for the Master Fund and 16,850 shares were purchased for the Account. As of December 12, 2016, the Master Fund held 3,145,714 shares, and the Account held 637,012 shares. Represents the weighted average purchase price of shares purchased in a series of open market transactions on the transaction date at prices ranging from $10.95 to $11.00 per share. The Reporting Persons undertake to provide, upon request by the staff of the Securities and Exchange Commission, the issuer, or a security holder of the issuer, full information regarding the number of shares purchased at each price. Of these shares, 17,712 were purchased for the Master Fund and 3,589 shares were purchased for the Account. As of December 13, 2016, the Master Fund held 3,163,426 shares, and the Account held 640,601 shares. /s/ CORMORANT GLOBAL HEALTHCARE GP, LLC By: Bihua Chen, Managing Member 2016-12-14 /s/ CORMORANT ASSET MANAGEMENT, LLC By: Bihua Chen, Managing Member 2016-12-14 /s/ CORMORANT GLOBAL HEALTHCARE MASTER FUND, LP By: Cormorant Global Healthcare GP, LLC, its General Partner By: Bihua Chen, Managing Member 2016-12-14 /s/ Bihua Chen 2016-12-14