Table 4 shows the risk of leukopenia according to NUDT15 and TPMT genotypes. NUDT15 variant types were strongly associated with developing leukopenia [10.9% wild-type as reference; 48.5% heterozygous variant genotype; 100% homozygous variant genotype, OR 3.44 (95% CI, 1.21–9.78)]. However, TPMT genotype was not associated with developing leukopenia (OR 0.58, 95% CI 0.07–4.96). Six patients with NUDT15 homozygous variant genotype developed severe early leukopenia with average reduction of 88.2% (range, 84–94%) from the baseline WBC count at 4 weeks (Fig 2 and S2 Table). The gene discussed is NUDT15; the disease is Decreased total leukocyte count.