This family of proteins is found in bacteria. Proteins in this family are approximately 140 amino acids in length. This family is the fourth subunit of the cytochrome c oxidase complex. This subunit does not have a catalytic capacity but instead, is required for assembly and/or stability of the complex.

1: Purification of a cytochrome bc-aa3 supercomplex with quinol oxidase activity from Corynebacterium glutamicum. Identification of a fourth subunity of cytochrome aa3 oxidase and mutational analysis of diheme cytochrome c1. Niebisch A, Bott M; J Biol Chem. 2003;278:4339-4346. PMID:12446663