UGT1A1 and Decreased total neutrophil count: Although 250 mg m−2 is a relatively low irinotecan dose, two patients in this study who developed grade 4 neutropenia within the first 2 weeks of cycle 1 were found to have decreased metabolism of SN-38 to its metabolite SN-38-glucuronide because of a UGT1A1*1/*28 and UGT1A1*28/*28 genotype, respectively.