Cyclophosphamide is a prodrug that must undergo activation by CYP2C9, CYP2C19, CYP3A4, and CYP3A5.4,5 CYP2B6 also participates in cyclophosphamide activation in the liver, but its role in the response to cyclophosphamide in cancer patients has not been proven yet.6 CYP2C8, CYP3A4, and CYP3A5 are major taxane-metabolizing enzymes.7,8 Roles of CYP1A2, CYP2A6, and CYP2C8 in 5-fluorouracil formation from a prodrug tegafur have been described as well.9CYP2C19 and CYP2D6 polymorphisms have recently been associated with therapeutic outcome of tamoxifen-treated breast carcinoma patients.10 The gene discussed is CYP3A4; the disease is breast carcinoma.