cytochrome P450 family 4, subfamily B and similar cytochrome P450s, including subfamilies A, T, X, and ZThis group is composed of family 4 cytochrome P450s from subfamilies A (CYP4A), B (CYP4B), T (CYP4T), X (CYP4X), and Z (CYP4Z). The CYP4A, CYP4X, and CYP4Z subfamilies are specific to mammals, CYP4T is present in fish, while CYP4B is conserved among vertebrates. CYP4As are known for catalyzing arachidonic acid to 20-HETE (20-hydroxy-5Z,8Z,11Z,14Z-eicosatetraenoic acid), and some can also metabolize lauric and palmitic acid. CYP4Bs specialize in omega-hydroxylation of short chain fatty acids and also participates in the metabolism of exogenous compounds that are protoxic including valproic acid (C8), 3-methylindole (C9), 4-ipomeanol, 3-methoxy-4-aminoazobenzene, and several aromatic amines. CYP4X1 is expressed at high levels in the mammalian brain and may play a role in regulating fat metabolism. CYP4Z1 is a fatty acid hydroxylase that is unique among human CYPs in that it is predominantly expressed in the mammary gland. Monophyly was not found with the CYP4T and CYP4B subfamilies, and further consideration should be given to their nomenclature. The CYP4B-like group belongs to the large cytochrome P450 (P450, CYP) superfamily of heme-containing proteins that catalyze a variety of oxidative reactions of a large number of structurally different endogenous and exogenous compounds in organisms from all major domains of life. CYPs bind their diverse ligands in a buried, hydrophobic active site, which is accessed through a substrate access channel formed by two flexible helices and their connecting loop.