This is a medical chart review study to collect information for patients who have been taking pyrimidine nucleosides for treatment of TK2 deficiency. Information from the time of onset of symptoms will be collected to describe the pre treatment course of TK2 deficiency.

Data to be collected from the time of onset of symptoms related to TK2 deficiency includes medical conditions and/or adverse events (AEs); these should include relationship to TK2 disease and/or pyrimidine nucleosides, as appropriate, as well as date of onset and severity, when available. When available, test reports may be obtained as well as available research biological samples (eg, blood or tissue samples that may be tested for biomarkers of disease and/or effects of medications to treat the mitochondrial disease).

Inclusion Criteria:

1. Signed informed consent by the patient or parent(s)/legally authorized representative (LAR) and/or assent by the patient (when applicable), unless the associated institutional review board (IRB) or ethics committee (EC) provides an appropriate consent waiver
2. Confirmed genetic mutation in the TK2 gene
3. Availability of medical records for each patient from the time of onset of symptoms
4. Patient has taken pyrimidine nucleos(t)ides (dCMP/dTMP and/or dC/dT) as substrate enhancement therapy for TK2 deficiency
5. Most recent patient visit at which efficacy and/or safety parameters were collected occurred between 01 June 2018 and 15 December 2018

Exclusion Criteria:

1. Presence of other genetic disease or polygenic disease

Patients with TK2 deficiency who have received treatment with pyrimidine nucleosides