Inclusion Criteria:

• 1. Low PDC activity in skin fibroblasts, blood lymphocytes or a muscle biopsy, below the reference range, and with valid internal controls to establish sample and assay integrity, and have had PDHA1 testing, and/or
• 2. A known pathogenic mutation of a gene associated with PDC deficiency.

Inclusion Criteria:

• 1. First or second degree relative of a primary subject for whom genetic testing indicates the presence of variants of unknown significance (VUS).

Exclusion Criteria:

• 1. Another chronic neurological disease (mitochondrial or non-mitochondrial) which is not considered likely to be related to PDC deficiency.
• 2. Inadequacy of needed blood or tissue sample and unwillingness or inability to submit such a sample.
• 3. Unwillingness to participate in the NAMDC Patient Data Registry and Biorepository protocol.
• 1. Inadequacy of needed blood sample and unwillingness or inability to submit such a sample.