Hephaestin is the gene affected in the sex-linked anemic (sla) mouse. These animals have a defect in the export of iron from intestinal enterocytes into the circulation and this implicates hephaestin in the basolateral transfer step of iron absorption. Hephaestin is homologous to the plasma copper-containing protein ceruloplasmin, and all residues involved in copper binding and disulfide bond formation in ceruloplasmin are conserved in hephaestin. Unlike ceruloplasmin, hephaestin is an integral membrane protein with a single trans-membrane domain. It is highly expressed throughout the small intestine, to a lesser extent in the colon, and at low levels in several other tissues. Surprisingly, most hephaestin appears to be located intracellularly in a perinuclear distribution. Like ceruloplasmin, hephaestin has a ferroxidase activity which is predicted to underlie its biological function. In addition, its expression is stimulated under iron deficient conditions. Analysis of the sla mouse has supported our model for the regulation of intestinal iron absorption whereby changes in systemic iron requirements alter the levels of basolateral transport components with subsequent regulation of brush border transport.