Members of this family are apparent metal cation efflux transporters. Architectural features include an average length of about 400 residues, with well conserved and highly hydrophobic N-terminal and C-terminal domains. The central region is highly variable in length and sequence, and rich in both Cys and His residues, as often seen in proteins produced in response to toxic concentrations of certain metals. The founding member, SO_0444, was shown to confer resistance to high levels of Cu and Zn ions. The best conserved region of the protein is a CSCG motif in the N-terminal region, found at least twice as in a selenocysteine-containing form, USCG.