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 NAD(+) as acceptor (By similarity). 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.Synonyms: Dudulin-2, Metalloreductase STEAP3, STEAP-3, Six-transmembrane epithelial antigen of prostate 3, Tumor suppressor-activated pathway protein 6, pH yde