Cytochrome P450 (CYP) is comprosed of a family of hemeprotein monooxygenases that catalyze reactions as diverse as the biosynthesis of steroid hormones, metabolism of fat-soluble vitamins, oxidation of unsaturated fatty acids, and metabolism of drugs, pollutants, and other xenobiotics (for a review, see ref. 1 ). About 55 CYP genes, grouped into 17 gene families, are present in the human genome. CYPs belonging to gene families 1, 2, and 3 are particularly active in drug and xenobiotic metabolism and are most abundantly expressed in the liver and other tissues that come in contact with foreign chemicals. Large interindividual variations in CYP expression and, consequently, CYP-dependent drug metabolism are seen in humans, as a result of both genetic and environmental factors.