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Menkes disease (MK; MIM 309400) is an X-linked recessive disorder that is usually lethal in early childhood. Menkes disease is caused by loss of function mutations in the ATP7A gene. ATP7A encodes a transmembrane copper-transporting P-type ATPase, a molecule involved in copper efflux from cells. This molecule transports copper to copper dependent enzymes synthesized within secretory compartments of cells. It is also involved in copper absorption from the gut. Classic Menkes disease findings include low serum copper and ceruloplasmin, growth retardation, bone abnormalities, joint laxity and sparse, kinky and steely hair (pili torti). Additional findings include hypopigmentation and laxity of skin, developmental delay, seizures, vascular abnormalities and autonomic dysfunction including hypotonia and hypothermia. The clinical phenotype is considered to reflect functional deficiencies of numerous copper requiring enzymes including: lysyl oxidase, tyrosinase, dopamine %u03B2-hydroxylase, peptidylglycine %u03B1-amidating monooxygenase, superoxide dismutase 1 and cytochrome c oxidase. Menkes disease is allelic to occipital horn syndrome (OHS; MIM 304150), and spinal muscular atrophy, distal, X-linked 3 (SMAX3; MIM 300489). OHS shares some clinical findings with Menkes disease but it is less severe. SMAX3 is distinct from both Menkes disease and OHS.