CYP2C9 is a crucial cytochrome P450 enzyme, which plays a significant role in the metabolism, by oxidation, of both xenobiotic and endogenous compounds. CYP2C9 makes up about 18% of the cytochrome P450 protein in liver microsomes. The protein is mainly expressed in liver, duodenum and small intestine. About 100 therapeutic drugs are metabolized by CYP2C9, including drugs with a narrow therapeutic index such as warfarin and phenytoin, and other routinely prescribed drugs such as acenocoumarol, tolbutamide, losartan, glipizide, and some nonsteroidal anti-inflammatory drugs. By contrast, the known extrahepatic CYP2C9 often metabolizes important endogenous compounds such as serotonin and, owing to its epoxygenase activity, various polyunsaturated fatty acids, converting these fatty acids to a wide range of biological active products.