UGT1A1 and Decreased total neutrophil count: It also recommends UGT1A1 genotype testing for the *28 and *6 alleles to determine the UGT1A1 metabolizing status, considering a reduction in the starting dose of irinotecan by at least one level for homozygous or compound heterozygous patients regarding the UGT1A1*28 and/or *6 alleles (UGT1A1*28/*28, *6/*6, or *6/*28 genotypes), closely monitoring patients with UGT1A1*28 or *6 alleles for neutropenia during and after treatment, and subsequent dosage modifications based on individual patient tolerance to treatment.