Mediates cellular uptake of transferrin-bound iron in a non-iron dependent manner. May be involved in iron metabolism, hepatocyte function and erythrocyte differentiation. Homodimer. Lacks the transmembrane domain. Probably intracellular. Predominantly expressed in liver. While the alpha form is also expressed in spleen, lung, muscle, prostate and peripheral blood mononuclear cells, the beta form is expressed in all tissues tested, albeit weakly. The disease is caused by variants affecting the gene represented in this entry. The variant Lys-172 found in hereditary hemochromatosis type III affects the putative initiation codon of the beta isoform thus preventing its translation. Belongs to the peptidase M28 family. M28B subfamily.