Inclusion Criteria:  Signed informed consent by the subject or parent(s)/LAR and/or assent by the subject (when applicable), unless the associated IRB or EC provides an appropriate consent waiver that allows review of medical records for this study. Confirmed genetic mutations in the TK2 gene. Availability of medical records or information pertaining to vital status at a minimum.  Exclusion Criteria:  Not applicable as this is a non-interventional study.