Occipital horn syndrome (OHS; MIM 304150) is an X-linked recessive disorder caused by mutations in the ATP7A gene. OHS, formerly known as Ehlers-Danlos syndrome IX, is characterized by occipital exostoses or ‘bony horns’ flanking both sides of the foramen magnum, skin laxity, easy bruisability, hyperextensible joints, bladder diverticula, ureteral obstruction, chronic diarrhea, head and face abnormalities, wormian bones and coarse hair. Some patients with OHS have developmental delay and mental retardation. 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. OHS is allelic to Menkes disease, a more serious disorder often resulting in death within the first three years of life. Menkes disease (MK; MIM 309400) is caused by loss of function mutations in the ATP7A gene, while patients with OHS display some residual ATP7A activity. Both Menkes disease and OHS are also allelic to spinal muscular atrophy, distal, X-linked 3 (SMAX3; MIM 300489), a peripheral motor neuron neuropathy.