Iron is an essential nutrient, and disorders of iron metabolism are common. Nonetheless, intestinal iron absorption and cellular iron transport are poorly understood. Biochemical approaches to elucidating these processes have yielded little in the past decade. As an alternative approach, we have begun to study spontaneous mouse mutants, that have inherited defects in key steps in iron transport. We have undertaken positional cloning of the gene responsible for microcytic anemia (gene symbol mk). This report describes the important characteristics of these mice, and our progress in studying them.