Mitochondrial disease is principally a chronic loss of cellular energy, where a failure to meet cellular energy demand results in a clinical phenotype. The clinical spectrum of mitochondrial disease is diverse, however, tissues where there is a high metabolic demand, such as the central nervous system (CNS) or heart, are typically affected. The broad clinical spectrum of mitochondrial dysfunction, coupled with the heterogeneity of mtDNA variation, makes the prevalence of mitochondrial DNA (mtDNA) difficult to calculate.Mitochondria are dependent upon the nuclear genome for the majority of the OXPHOS system and also for maintaining and replicating mtDNA as well as organelle network proliferation and destruction. To date, 92 structural OXPHOS subunit genes have been identified: 13 encoded by mtDNA and 79 encoded by the nuclear genome.