The Food and Drug Administration placed a partial clinical‑trial hold on Daiichi Sankyo and Merck & Co.’s global Phase 3 program for their investigational antibody‑drug conjugate after an unexpected cluster of deaths. Companies halted enrollment earlier and notified regulators; the FDA action formalizes safety review and limits ongoing dosing while investigators analyze cause. Daiichi and Merck cited a "higher‑than‑anticipated incidence" of deaths in the program; sponsors and the regulator are reviewing adverse‑event data, adjudications, and on‑study causes of death. The hold affects ongoing global enrollment and will delay pivotal timelines until safety signals are resolved. For oncology developers and investors, the hold underscores risk in ADC payload dosing and patient selection for high‑value registrational programs. Regulators evaluating clustered mortality typically require detailed root‑cause analyses, potential protocol amendments, and new safety monitoring before allowing resumed enrollment.