From Bioblast
Cytochrome c oxidase or Complex IV (CIV) is the terminal oxidase of the mitochondrial ETS, reducing oxygen to water, with reduced cytochrome c as a substrate. CIV is frequently abbreviated as COX or CcO. It is the 'ferment' (Atmungsferment) of Otto Warburg, shown to be related to the cytochromes by David Keilin.