The human cytochrome P450 family 2, subfamily B (CYP2B) consists of only one functional member CYP2B6, which shows broad substrate specificity and plays a key role in the metabolism of many clinical drugs, environmental toxins, and endogenous compounds. Rodents have multiple functional CYP2B proteins; mouse subfamily members include CYP2B9, 2B10, 2B13, 2B19, and 2B23. CYP2B enzymes are highly inducible by chemicals that interact with the constitutive androstane receptor (CAR) and/or pregnane X receptor (PXR), such as rifampicin and phenobarbital. The CYP2B subfamily 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.