Summary Genetic disorders of trace element transport are now known in humans, mice, dogs and cattle. Those involving copper have been known longest and are best known clinically. Effects due to copper deficiency are seen in Menkes' disease, in X-linked cutis laxa and in the X-linked series of mottled mutants in the mouse. Copper accumulation is also harmful, causing damage initially to the liver and later to the kidneys and brain in Wilson's disease, in some Bedlington terriers and in toxic milk mice. Zinc deficiency is seen in acrodermatitis enteropathica and in premature babies born to women who seem to secrete milk that is zinc-deficient, as is seen in lethal milk mice. Study of animal mutants, especially mutant mice, is helpful in understanding the human diseases and identification of the basic defects in trace element transport in these diseases is improving knowledge relevant to trace element nutrition.