The U.S. Food and Drug Administration approved UCB’s Kygevvi for thymidine kinase 2 (TK2) deficiency in adults and children with symptom onset by age 12, marking the first approved therapy for this ultra‑rare mitochondrial disease. The decision follows Phase II data showing survival benefit against an external control cohort, according to MedCity News and UCB releases. UCB framed the approval as validation of its renewed rare‑disease focus after restructuring. Regulatory reviewers relied on comparisons with untreated natural history cohorts; such external‑control approaches are increasingly used for rare conditions but can invite debate about bias and evidentiary standards. The approval establishes a commercial and scientific precedent for therapies targeting extremely low‑incidence genetic disorders.