The FDA approved UCB’s KYGEVVI (doxecitine and doxribtimine) as the first therapy for thymidine kinase 2 deficiency (TK2d), an ultra‑rare, often fatal mitochondrial disease. Approval relied on a pooled evidence package including a Phase II trial, retrospective chart reviews, and expanded access data showing treated patients had markedly better survival versus matched external controls—an 86% reduction in overall risk of death from treatment start was reported. KYGEVVI supplies pyrimidine nucleosides intended to restore mitochondrial DNA copy number; regulators accepted survival comparisons against an external natural‑history cohort.