Clinical history consistent with the diagnosis of mitochondrial disease, and molecular genetic diagnosis. To ensure consistency with other trials performed in mitochondrial disease, investigators will also ensure that participants meet the same set of previously published criteria. These include clinical features consistent with primary mitochondrial disease and molecular genetic proof of a pathogenic mutation in mtDNA or nDNA in a gene known to be associated with dysfunction of complexes I-V of the respiratory chain. Specifically, eligible participants must have defined mtDNA or nDNA mutations affecting subunits or assembly of these complexes that are associated with known clinical/pathological features, such as chronic progressive external ophthalmoplegia (CPEO), Kearns-Sayre, mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), mitochondrial encephalopathy and ragged red fibers (MERRF), neuropathy, ataxia and retinitis pigmentosa (NARP) or Leigh syndrome (45). Investigators will explicitly include individuals with Friedreich's Ataxia (46), a mutation in the mitochondrial protein frataxin, and those with mutations in respiratory chain complex II protein, succinate dehydrogenase (SDH).