Cytochrome b5 type B (CYB5B) is a membrane of the cytochrome b5 family. It contains 1 cytochrome b5 heme-binding domain. Cytochrome b5 is a membrane bound hemoprotein which function as an electron carrier for several membrane bound oxygenases. In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III also known as the bc1 complex or ubiquinol-cytochrome c reductase.