3. All subjects will have either suspected mitochondrial dysfunction as assessed by clinical evaluation, mitochondrial dysfunction as defined by the presence of significant abnormalities in their buccal mitochondrial respiratory complex activities (i.e., with either respiratory complex I or complex IV deficiencies) or have significantly aberrant specific activity ratios. Subjects with significant deficiencies in either muscle or skin fibroblast respiratory activities will also be included in those cases if buccal mitochondrial respiratory enzyme activity testing has not been performed.