Cytochrome c oxidase subunit III (COX3) is an enzyme that in humans is encoded by the MT-CO3 gene. It is one of main transmembrane subunits of cytochrome c oxidase. Cytochrome c oxidase subunit III is also one of the three mitochondrial DNA (mtDNA) encoded subunits (MT-CO1, MT-CO2, MT-CO3) of respiratory complex IV. Variants of MT-CO3 have been associated with isolated myopathy, severe encephalomyopathy, Leber hereditary optic neuropathy, mitochondrial complex IV deficiency, and recurrent myoglobinuria .