Endosomal ferrireductase required for efficient transferrin-dependent iron uptake in erythroid cells. Participates in erythroid iron homeostasis by reducing Fe(3+) to Fe(2+). Can also reduce of Cu(2+) to Cu(1+), suggesting that it participates in copper homeostasis. Uses NADP(+) as acceptor. May play a role downstream of p53/TP53 to interface apoptosis and cell cycle progression. Indirectly involved in exosome secretion by facilitating the secretion of proteins such as TCTP. Homodimer. Interacts with BNIP3L, MYT1, RHBDL4/RHBDD1 and TCTP. Localizes to vesicular-like structures at the plasma membrane and around the nucleus. Expressed in adult bone marrow, placenta, liver, skeletal muscle and pancreas. Down-regulated in hepatocellular carcinoma. By p53/TP53. Proteolytically cleaved by RHBDL4/RHBDD1. RHBDL4/RHBDD1-induced cleavage occurs at multiple sites in a glycosylation-independent manner. Glycosylated. The disease is caused by variants affecting the gene represented in this entry. Belongs to the STEAP family. Was initially thought to have tumor suppressor function in prostate cancer. However, it was shown that it is probably not the case (PubMed:12866033).