Severe neutropenia was observed in 22 (91.7%) of the patients with low WBC count at the baseline, 30 (60%) of the patients with UGT1A1 heterozygous for UGT1A1*28 or UGT1A1*6 polymorphism, and 10 (100.0%) of the patients had both of the above factors. The gene discussed is UGT1A1; the disease is neutropenia.