Cytochrome P450s (CYPs) are a large superfamily of enzymes found in almost all living organisms (Nelson et al., 1993); their ubiquity may reflect their ancient origin and physiological importance. CYPs catalyse a wide range of reactions involving both endogenous and exogenous substrates; they are involved in the biosynthesis and catabolism of steroids, retinoids, prostaglandins and fatty acids, and the detoxification of exogenous substrates including drugs and insecticides