Abstract
Doxecitine and doxribtimine (KYGEVVI(®)) are pyrimidine nucleosides developed by UCB for the treatment of thymidine kinase 2 deficiency (TK2d). In November 2025, doxecitine and doxribtimine received its first approval in the USA for the treatment of TK2d in adult and pediatric patients with an age of symptom onset on or before 12 years. This article summarizes the milestones in the development of doxecitine and doxribtimine leading to this first approval for TK2d.