A number of different cytochrome P-450 proteins are found in human liver and other tissues. These enzymes oxidize drugs and carcinogens as well as endogenous chemicals such as steroids and eicosanoids. As Peter Guengerich explains, activities of individual cytochrome P-450 enzymes vary among individuals as a result of both genetic and environmental influences; in some cases, the mechanisms are known. Such variation can have major influences in determining drug toxicity, inborn errors of steroid metabolism, and possibly cancer risk.