Day One Pharmaceuticals agreed to acquire Mersana Therapeutics for $129 million up front and up to $285 million in total contingent value tied to Emi‑Le (emiltatug ledadotin), a B7‑H4‑targeting antibody‑drug conjugate (ADC). The transaction follows Mersana’s strategic focus on Emi‑Le after earlier pipeline cuts and clinical setbacks. Day One positioned the deal as an opportunistic acquisition that augments its oncology portfolio with a late‑stage ADC program; Mersana shareholders will receive upfront cash plus contingent payments tied to development and commercial milestones. The arrangement transfers Emi‑Le’s ongoing expansion trials and related assets into Day One’s development plan. The back‑loaded structure reduces immediate cash strain while preserving upside for Mersana stakeholders if Emi‑Le meets clinical and commercial goals.