CYP2D6 accounts for only a small percentage of total hepatic CYPs (<2%), but it metabolizes approximately 25% of clinically used drugs (>100) with significant polymorphisms. A number of drugs acting on the central nervous system and cardiovascular system are substantially metabolized by CYP2D6. The enzyme also utilizes hydroxytryptamines and neurosteroids as endogenous substrates. In addition, CYP2D6 metabolizes procarcinogens and neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, and indolealkylamines. Typical CYP2D6 substrates are usually lipophilic bases with a planar hydrophobic aromatic ring and a nitrogen atom which can be protonated at physiological pH, but several atypical substrates such as spirosulfonamide and pactimibe do not contain a basic nitrogen atom. The structure of human CYP2D6 has been recently determined and shows the characteristic CYP fold as observed in other members of the CYP superfamily, with a well-defined active site cavity above the heme group with a volume of approximately 540 A(3). CYP2D6 is largely uninducible by prototypical CYP inducers such as phenobarbital, rifampin and dexamethasone, but it is regulated by hepatocyte nuclear factor-4alpha, a nuclear receptor. CYP2D6 is subject to inhibition by a number of drugs and this may provide an explanation for numerous clinical drug interactions. CYP2D6 has an important role in drug development and it is a common practice for pharmaceutical industry nowadays to a great extent screen drug candidates early in development as possible CYP2D6 substrates and/or inhibitors and drop such candidates where they have alternatives. This candidate selection might eventually lead to a less prominent role of this enzyme in the future for drug metabolism and less interindividual variability in drug exposure and minimize potentially adverse drug interactions. Further studies are warranted to delineate the molecular mechanisms involved in the function and regulation of CYP2D6.