Menkes syndrome is an X-linked genetic copper deficiency that is usually fatal in early childhood. Milder variants exist, including occipital horn syndrome, which is primarily a connective tissue disorder. Mutations of the mottled locus in mice produce a wide range of copper-deficient phenotypes that are good models for human diseases. Understanding the nature of the defects has been greatly increased as a result of the identification of the gene affected in Menkes syndrome. The gene spans approximately 140 kilobases, contains 23 exons, and encodes a copper-transporting ATPase termed MNK that is thought to be involved in copper efflux from cells. More recent studies show that MNK is located primarily in the trans-Golgi compartment of Chinese hamster ovary cells. Copper-resistant cells overexpress MNK and can efflux more copper than parental cells, consistent with the copper efflux role proposed for MNK. Patients with Menkes syndrome are predicted to have little or no MNK activity, whereas patients with occipital horn syndrome have less severe mutations and some residual MNK activity is predicted. Similarly, the mottled mice mutants have a range of mutations in the MNK gene homologue. Complete loss of MNK, however, produces a fetal lethal phenotype in mice. A model is proposed to explain the wide range of phenotypes exhibited by the different mouse mutants. Further research into the cell biology of copper transport is expected to reveal more about the molecular basis of copper homeostasis.