Cytochrome P450 oxidase (commonly abbreviated CYP) is a generic term for a large number of related, but distinct, oxidative enzymes important in vertebrate physiology. The cytochrome P450 mixed-function monooxygenase system is probably the most important element of Phase I metabolism in mammals. P450s are membrane-bound, either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells where they metabolise thousands of endogenous and exogenous compounds. In the liver, these substrates include toxins, drugs, and other unneeded and potentially harmful molecules. Humans have 18 families of cytochrome P450 genes and 43 subfamilies; the CYP1 family is involved in drug metabolism and includes 3 subfamilies, 3 genes and 1 pseudogene. The CYP2 family are part of the microsomal drug metabolising system that is responsible for oxidation of many therapeutic agents as well as steroids, fatty acids and many other endogenous substances. Cytochrome P450 3A4 is abundantly expressed in liver and small intestine and is inducible by barbiturates, glucocorticoids and rifampicin. P6 is highly specific for members of the CYP 3A gene family and cross reacts with CY 3A4 in human, CYP 3A in rat and potentially CYP 3A11 (based on inducibility) in mice.