The cytochrome P450 family 1 (CYP1 or Cyp1) is composed of three functional human members: CYP1A1, CYP1A2 and CYP1B1, which are regulated by the aryl hydrocarbon receptor (AhR), ligand-activated transcriptional factor that dimerizes with AhR nuclear translocator (ARNT). CYP1 enzymes are involved in the metabolism of endogenous hormones, xenobiotics, and drugs. Included in the CYP1 family is CYP1D1 (cytochrome P450 family 1, subfamily D, polypeptide 1), which is not expressed in humans as its gene is pseudogenized due to five nonsense mutations in the putative coding region, but is functional in in other organisms including cynomolgus monkey. Zebrafish CYP1D1 expression is not regulated by AhR. The CYP1 family 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.