The FDA approved UCB’s KYGEVVI (doxecitine and doxribtimine), the first therapy for thymidine kinase 2 deficiency (TK2d), an ultra-rare mitochondrial disease. Approval was supported by a Phase II study, retrospective chart reviews and an expanded-access program totaling 82 treated patients compared against matched external controls. Data showed an 86% reduction in overall risk of death from treatment start versus matched untreated patients. Patient groups and UCB characterized the approval as a major milestone for a previously untreatable condition.