There are three recognized X-linked copper deficiency disorders in humans: classical Menkes disease (MD), mild Menkes disease and occipital horn syndrome (OHS, also known as X-linked cutis laxa). Since the features of these diseases are so distinct, it was not clear until recently whether the phenotypes were due to mutations in the same gene, or whether OHS in particular is due to mutations in a gene on the X-chromosome, closely linked to the Menkes locus (Danks, 1995). Molecular analysis has now demonstrated that MD, mild Menkes and OHS are indeed due to allelic mutations of the gene affected in Menkes disease (MNK or ATP7A), however, the basis for the phenotypic differences is still not fully understood. A similar range of phenotypes is also found in the mottled mice mutants, and these are discussed below.