UGT1A1 and Decreased total neutrophil count: Eleven patients (39.2%) carrying the homozygous UGT1A1*28 allele developed grade ≥ 3 neutropenia despite upfront reduced irinotecan doses, as compared to 19.6% of heterozygous or wild-type patients treated with full doses (ORvs*28/*28 genotype = 0.38; 95% CI: 0.14–1.03; p = 0.058).