Patients with specific polymorphisms in UGT1A1 (UGT1A1*28) have impaired metabolism of SN38 and are predisposed to the major toxicities of irinotecan, which are diarrhoea and myelosuppression, particularly neutropenia (Innocenti et al, 2004; O’Dwyer and Catalano, 2006). Here, UGT1A1 is linked to neutropenia.