Day One Biopharmaceuticals closed on the acquisition of Mersana Therapeutics pursuant to an agreement dated November 13, 2025.[1] Mersana shareholders will receive an immediate payment of $25 per share in cash, a value of $129 million.[1][2] They may additionally receive up to $30.25 per share in cash if Emi-Le's drug achieves certain clinical, regulatory and commercial milestones, bringing the total transaction value to $285 million.[1][2] The immediate settlement represents a premium of 181.85% over the last closing share price.[1] Mersana is a clinical-stage biopharmaceutical company developing antibody-coupled drugs (ADCs) through its Dolasynthen and Immunosynthen platforms for the treatment of cancer.[1] The main objective of the acquisition is to acquire the drug Emi-Le, which is in the initial phase of testing and which could be used to treat adenoid cystic carcinoma, which has a high number of unmet medical needs.[4] The transaction was expected to close by the end of January 2026.[1][2]