Treatment of TK2 Deficiency With Thymidine and Deoxycytidine

Inclusion Criteria: * Genetically confirmed diagnosis of TK2 deficiency * Deemed by principle investigator to be symptomatic with TK2 deficiency * Single gene disease; absence of polygenic disease * Hematocrit within normal range for age group * Patient or patient's guardian able to consent and comply with protocol requirements * Presence of caregiver to ensure study compliance (if needed) * Abstention from use of all pill-form dietary supplements and non-prescribed medications (except as allowed by the investigator) * Abstention from use of other investigational medications or other medications according to the study investigator Exclusion Criteria: * Clinical history of bleeding or abnormal prothrombin time (PT)/partial thromboplastin time (PTT) * Hepatic insufficiency with liver function tests (LFTs) greater than two times normal * Renal insufficiency requiring dialysis * Any other concurrent inborn errors of metabolism * Severe end-organ hypo-perfusion syndrome secondary to cardiac failure resulting in lactic acidosis