Complex IV or cytochrome c oxidase (COX) catalyses the oxidation of cytochrome c and the reduction of oxygen to water, coupled to proton translocation. Mammalian cIV contains 13 subunits. MT-CO1 is the largest catalytic subunit. MT-CO2 is the second core subunit. MT-CO3, the third core subunit has no direct catalytic role. The supernumerary subunits do not seem to have a catalytic role, but are thought to be important for the stabilization and regulation of the catalytic core. Complex IV is the only OXPHOS complex containing tissue-specific and developmentally regulated isoforms.