Menkes disease is caused by mutations in the copper-transporting P(1B)-type ATPase ATP7A. ATP7A has a dual function: it serves to incorporate copper into copper-dependent enzymes, and it maintains intracellular copper levels by removing excess copper from the cytosol. To accomplish both functions, the protein traffics between different cellular locations depending on copper levels. The mechanism for sensing the concentration of copper, for trafficking, as well as the details of the mechanism of copper translocation across the membrane are unknown.