Cytochrome b5 (CYB5A) is a membrane bound hemoprotein which function as an electron carrier for several membrane bound oxygenases. CYB5A contains one cytochrome b5 heme-binding domain and has two isoforms produced by alternative splicing. Isoform 1 is a sngle-pass membrane protein. Isoform 2 is located in cytoplasm. The defects in CYB5A can result in type IV hereditary methemoglobinemia.