The protein encoded by this gene is the terminal enzyme of the respiratory chain, coupling the transfer of electrons from cytochrome c to molecular oxygen, with the concomitant production of a proton electrochemical gradient across the inner mitochondrial membrane. In addition to 3 mitochondrially encoded subunits, which perform the catalytic function, the eukaryotic enzyme contains nuclear-encoded smaller subunits, ranging in number from 4 in some organisms to 10 in mammals. It has been proposed that nuclear-encoded subunits may be involved in the modulation of the catalytic function. This gene encodes one of the nuclear-encoded subunits.