Mediates cellular uptake of transferrin-bound iron in a non-iron dependent manner. May be involved in iron metabolism, hepatocyte function and erythrocyte differentiation. Predominantly expressed in liver. Also expressed in kidney, spleen, brain, lung, heart and muscle with very low expression in kidney, muscle and heart. First expressed between embryo days 8 and 11. In the liver, expression increases during development from embryo day 13 to adulthood while, in the spleen, levels remain constant throughout development. Down-regulated during erythrocyte differentiation. Expression unchanged by cellular iron status. Lacks most of the extracellular domain. Belongs to the peptidase M28 family. M28B subfamily.