Erythroid cells of the bone marrow, the most avid consumers of iron in the body, acquire ferric (Fe³⁺) iron exclusively via the transferrin cycling pathway. A long-standing fundamental molecular question of how ferric iron is handled in this pathway has been recently resolved by the identification of Steap3 (six-transmembrane epithelial antigen of the prostate 3) as an endosomal ferrireductase needed for efficient utilization of transferrin-delivered iron. Further characterization of Steap3 and other Steap proteins reveals a possible greater role of Steap proteins in iron and copper metabolism.