The European Medicines Agency (EMA) issued an opinion on the granting of marketing authorization for the drug Kygevvi (doxecitine and doxribtimine) in patients with confirmed thymidine kinase 2 deficiency (TK2d), whose symptoms appeared before 12 years of age[4]. TK2d is a rare, life-threatening genetic disease affecting less than 1 in 1,000,000 people, with no approved drug to date[4]. Kygevvi consists of pyrimidine nucleosides, which are thought to compensate for reduced TK2 activity by promoting mitochondrial DNA production in muscle cells[4]. The EMA's opinion is based on data from a retrospective chart analysis and a phase 2 clinical trial on 39 patients with disease onset under 12 years of age[4]. After treatment with Kygevvi, 84% of patients regained one or more motor milestones, indicating improvement in motor function[4]. The most common side effects include diarrhea, vomiting, and abdominal pain[4]. Authorization was recommended under exceptional circumstances with an annual review and the obligation to conduct a new study to confirm safety and efficacy[4]. Treatment is intended for adults and children with symptoms up to 12 years of age[4].