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.