Cytochrome P450-dependent monooxygenases are the key enzymes of phase I reactions. They are especially abundant in the liver and play an important role in xenobiotic metabolism. In the hepatocytes, P450s are found in two different compartments, in which they have two different electron transfer chains. P450 enzymes that belong to different families also have different gene regulatory mechanisms. Many of these genes can be turned on or induced by a chemical signal, potentially related to the role of P450 in detoxification. Some P450s can activate procarcinogens to carcinogens, so their induction may be a cancer risk. The activity and inducibility of particular P450 isoforms differ in particular zones of the hepatic acini. They are also dependent on many physiological factors, including the age and sex of an individual. The term “cytochrome P450-dependent monooxygenase system” (CYP-450) denotes a complex of integral proteins situated in the microsomal membrane, nuclear capsule, plasma lemma, and mitochondria. References and appendix