The cytochrome c oxidase (COX) family of proteins function as the final electron donor in the respiratory chain to drive a proton gradient across the inner mitochondrial membrane, ultimately resulting in the production of water. The mammalian COX apoenzyme is a dimer, with each monomer consisting of 13 subunits, some of which are mitochondrial and some of which are nuclear. Localized to the intermembrane space, COX6b2 (Cytochrome c oxidase subunit 6B2), also known as Cytochrome c oxidase subunit VIb isoform 2 and Cancer/testis antigen 59, is a 88 amino acid mitochondrial protein that is responsible for joining the two COX monomers to form the COX dimer. COX6b2 is specifically expressed in testis and is found to be upregulated in certain cancer cell lines.