Day One Biopharmaceuticals, Inc. (NasdaqGS:DAWN) entered into an Agreement and Plan of Merger to acquire Mersana Therapeutics, Inc. (NasdaqGS:MRSN) from BCIP Life Sciences Associates, LP, BCLS II Investco, LP and Bain Capital Life Sciences Fund II, L.P., funds managed by Bain Capital Life Sciences Investors, LLC and others for approximately $130 million on November 12, 2025. Under the terms of the acquisition, $25 in cash per share will be paid as an Upfront Consideration and $30.25 in cash per share will paid as contingent value rights, upon the achievement of certain specified milestones in accordance with the terms and subject to the conditions of a contingent value rights agreement. Mersana?s executive officers, directors and certain stockholders affiliated with Bain Capital Life Sciences, holding in the aggregate approximately 8.5% of Mersana?s outstanding shares of common stock, have signed tender and support agreements to tender their shares in the tender offer. Under the terms of the definitive merger agreement, the tender offer is required to be commenced within 10 business days of November 12, 2025. Any shares not tendered in the tender offer will be acquired in a second-step merger for the same consideration as paid in the tender offer on a per share of Mersana common stock basis. The Offer will remain open for 20 business days, subject to extension under certain circumstances. Upon the closing of the transaction, Mersana will become a wholly-owned subsidiary of Day One and shares of Mersana?s common stock will no longer be listed on any exchange. In case of termination of transaction, Mersana Therapeutics, Inc. will pay a termination fee of $5.6 million to Day One Biopharmaceuticals, Inc. The transaction will be financed through the existing cash resources of Day One Biopharmaceuticals, Inc.

The transaction is subject to the satisfaction of customary closing conditions, including that a majority of Mersana?s shares of common stock are validly tendered in the tender offer and not validly withdrawn and the receipt of certain U.S. regulatory approvals, HSR act approval, court approval. The deal has been unanimously approved by the board of directors of Mersana Therapeutics, Inc and closing is expected to occur by the end of January 2026. As on January 5, 2026, tender offer expired and as a result 3,029,135 Shares were validly tendered.

Cowen Inc. acted as financial advisor to Mersana Therapeutics, Inc. Christopher D. Barnstable-Brown, Stuart M. Falber, Andrew Langworthy, Andrea Sorrentino, Sarah Matchett, Steve Barrett, Caroline Dotolo, Ciara Baker, David Strong, Bruce Manheim, Dominic Vote, Kirk Nahra, Mat Trachok, Ben Kelsey, Ali Jessani and Carla Gilbertson of Wilmer Cutler Pickering Hale and Dorr LLP acted as legal advisors to Mersana Therapeutics, Inc. Steve Albertson, William R. Skinner, Marshall Mort, Amy Manning, Stefano Quintini, Julia Forbess, Robert A. Freedman, David K.Michaels and Effie Toshav of Fenwick & West LLP acted as legal advisor to Day One Biopharmaceuticals, Inc. Gordon Dyal & Co. Advisory Group LP acted as exclusive financial advisor to Day One Biopharmaceuticals, Inc. Georgeson LLC acted as information agent and Computershare Trust Company, National Association acted as transfer agent to Day One Biopharmaceuticals, Inc. Computershare Trust Company, National Association acted as transfer agent to Mersana Therapeutics, Inc. Tina Xiang, Jamie M. G leason-Bremer, Maya Skubatch and Julia L. Minitti of Wilson Sonsini Goodrich & Rosati acted as due diligence provider to Day One Biopharmaceuticals, Inc.