This short presumed domain is about 50 amino acid residues long. It often contains two cysteines that may be functionally important. This domain is found in copper transporting ATPases, some phenol hydroxylases and in a set of uncharacterised membrane proteins including Swiss:Q9CNI0. This domain is named after three of the most conserved amino acids it contains. The domain may be metal binding, possibly copper ions. This domain is duplicated in some copper transporting ATPases.