The FDA approved UCB’s KYGEVVI (combination of pyrimidine nucleosides) as the first treatment for thymidine kinase 2 deficiency (TK2d), an ultra‑rare, often fatal mitochondrial myopathy. Approval was based on a Phase II study, retrospective chart reviews and expanded access data totaling 82 treated patients and matched external controls, showing an 86% reduction in mortality risk from treatment start. Regulators accepted evidence from non‑randomized sources given the disease’s rarity and poor natural history; the approval provides the first disease‑modifying option for many pediatric and adult patients. Patient advocates hailed the decision as transformative for a previously untreatable community. UCB said KYGEVVI aims to restore mitochondrial DNA copy number by supplying deoxynucleosides and will pursue post‑marketing commitments to further characterize long‑term benefit and safety.