UGT1A1 and neutropenia: Clinically, the use of irinotecan is limited by diarrhoea and neutropenia with potential impact on dose intensity as well as patient acceptability; low activity of the SN-38 metabolising enzyme UGT1A1 is associated with a greater risk of diarrhoea and myelosuppression [6], and in 2005 the US FDA recommended irinotecan dosing be modified in patients carrying the UGT1A1*28 polymorphism [2,7].