X0306 4 2020-05-22 1 0001383701 Histogen Inc. HSTO 0001714828 Marshall Keith W C/O HISTOGEN INC. 10655 SORRENTO VALLEY ROAD, SUITE 200 SAN DIEGO CA 92121 0 1 0 0 EVP, COO & CFO Common Stock 2020-05-22 4 M 0 38500 0.00 A 38500 D Common Stock 2020-05-22 4 F 0 18615 0.56 D 19885 D Restricted Stock Units 2020-05-22 4 M 0 38500 0.00 D Common Stock 38500 0 D Each restricted stock unit represents a contingent right to receive one share of the Issuer's common stock. Pursuant to the Agreement and Plan of Merger and Reorganization (the "Merger Agreement") by and among Conatus Pharmaceuticals Inc. ("Conatus"), Chinook Merger Sub, Inc. and Histogen Inc., all outstanding Conatus restricted stock units vested in full and were settled in shares of common stock prior to the closing of the merger contemplated by the Merger Agreement. All share amounts in this Form 4 reflect the 10 for 1 reverse stock split implemented by Conatus Pharmaceuticals Inc. on May 26, 2020 in connection with the Merger Agreement. /s/ Steven J. Mento, Attorney-in-Fact for Keith W. Marshall 2020-05-27