Day One Biopharmaceuticals agreed to acquire Mersana Therapeutics, paying $129 million upfront and up to $285 million including milestones, in a backloaded deal focused on Mersana’s B7‑H4–targeting ADC Emi‑Le. The transaction transfers clinical development responsibilities and consolidates Emi‑Le into Day One’s oncology portfolio. Mersana pivoted earlier this year to prioritize Emi‑Le after pipeline and financial pressures; Day One plans to leverage its clinical and commercial capabilities to advance the ADC’s expansion cohorts in breast and other solid tumors. The structure uses contingent value rights to bridge valuation gaps tied to clinical milestones. The acquisition demonstrates a triage pattern in biotech M&A where smaller firms with a single promising asset find buyers willing to pay modest upfronts with milestone upside. For investors, the deal balances downside protection with potential upside if Emi‑Le meets clinical endpoints.