Day One Biopharmaceuticals agreed to acquire Mersana Therapeutics for $129 million upfront with additional contingent payments that lift the deal value to as much as $285 million, centered on Mersana’s lead B7‑H4 antibody‑drug conjugate Emi‑Le (emiltatug). The transaction follows Mersana’s decision earlier this year to focus its cash and pipeline on Emi‑Le after a pivotal setback for a different ADC program. Day One said the acquisition complements its pediatric oncology portfolio and provides commercial capabilities for Emi‑Le if clinical development succeeds. Mersana shareholders will receive $25 per share in cash plus contingent value rights tied to development, regulatory and commercial milestones. Management framed the deal as a portfolio consolidation that preserves Emi‑Le’s development while extending Day One’s oncology reach.