Interestingly, the statistical significance of 6-MP-induced myelotoxicity was observed in heterozygous TPMT*1/*3C after excluding the patients who had variant NUDT15. Patients who were heterozygous TPMT*1/*3C were significantly associated with an increased risk of 6-MP-induced leukopenia (WBC < 2000 cell/mm3) with an OR of 4.10 (95% CI: 1.06–15.95 (p = 0.031)), and 6-MP-induced neutropenia (ANC < 500 cell/mm3) with an OR of 4.17 (95% CI: 1.25–13.91 (p = 0.014)) when compared with homozygous wild type (TPMT*1/*1) patients during eight weeks of maintenance therapy. This evidence concerns the gene NUDT15 and neutropenia.