The function of CYBRD1 (cytochrome b reductase 1, Ensembl gene identifier ENSG00000071967) is as follows. Plasma membrane reductase that uses cytoplasmic ascorbate as an electron donor to reduce extracellular Fe(3+) into Fe(2+) (PubMed:30272000). Probably functions in dietary iron absorption at the brush border of duodenal enterocytes by producing Fe(2+), the divalent form of iron that can be transported into enterocytes (PubMed:30272000). It is also able to reduce extracellular monodehydro- L-ascorbate and may be involved in extracellular ascorbate regeneration by erythrocytes in blood (PubMed:17068337). May also act as a ferrireductase in airway epithelial cells (Probable). May also function as a cupric transmembrane reductase (By similarity). {ECO:0000250|UniProtKB:Q925G2, ECO:0000269|PubMed:17068337, ECO:0000269|PubMed:30272000, ECO:0000305|PubMed:16510471}.