X0306 4 2021-02-22 0 0001829802 Sensei Biotherapeutics, Inc. SNSE 0001843594 Peyer James C/O SENSEI BIOTHERAPEUTICS, INC. 1405 RESEARCH BLVD, SUITE 125 ROCKVILLE MD 20850 1 0 1 0 Common Stock 2021-02-22 4 P 0 9802 18.88 A 4712494 I See footnote The price reported is a weighted average sales price. These shares were sold in multiple transactions at prices ranging from $18.55 to $19.50, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set herein. These shares are held by Cambrian Biopharma, Inc., a Delaware corporation ("Cambrian"). The Reporting Person is the Chief Executive Officer of Cambrian and in such capacity may direct the voting and disposition of the shares held by Cambrian, subject in certain instances to the approval of the Cambrian Board of Directors. Mr. Peyer disclaims beneficial ownership of such shares. /s/ Mark Ballantyne, Attorney-in-fact 2021-02-24