The function of ENSG00000100243 (CYB5R3, cytochrome b5 reductase 3) is as follows. Catalyzes the reduction of two molecules of cytochrome b5 using NADH as the electron donor. {ECO:0000269|PubMed:10807796, ECO:0000269|PubMed:1400360, ECO:0000269|PubMed:15953014, ECO:0000269|PubMed:1898726, ECO:0000269|PubMed:2019583, ECO:0000269|PubMed:8119939, ECO:0000269|PubMed:9639531}.