This is a retrospective, chart-review study. The primary goal is to use data on survival and other related information to support a comprehensive evaluation of patients with Thymidine Kinase 2 deficiency (TK2d) who have not been treated with pyrimidine nucleos(t)ides including deoxycytidine monophosphate/deoxythymidine monophosphase (dCMP/dTMP), and deoxycytidine/deoxythymidine (dC/dT), and/or MT1621(dC/dT), and those that have received one or more of these treatments. The secondary goals of this study will focus on describing the participant's clinical course and treatment experience.
