7-Ethyl-10-hydroxycamptothecin (SN-38) is the active metabolite of irinotecan (an analog of camptothecin – a topoisomerase I inhibitor); it is 1000 times more active than irinotecan itself. In vitro cytotoxicity assays show that the potency of SN-38 relative to irinotecan varies from 2- to 2000-fold. SN38 is metabolized via glucoronidation by UGT1A1. The variant of UGT1A1 in ~10% of Caucasians which leads to poor metabolization of irinotecan predicts irinotecan toxicity, as it cannot be excreted from the body in its SN-38 form. SN-38 is lost into the bile and feces. It can cause the symptoms of diarrhoea and myelosuppression experienced by ~25% of the patients administered irinotecan.