Cytochrome b5 type A encoded by CYB5A is a membrane-bound cytochrome that reduces ferric hemoglobin (methemoglobin) to ferrous hemoglobin, which is required for stearyl-CoA-desaturase activity. Defects in CYB5A are a cause of type IV hereditary methemoglobinemia. Three transcript variants encoding different isoforms have been found for CYB5A.