A nonsynonymous single nucleotide polymorphism (SNP), rs116855232 (the c.415C > T variant), which gives rise to the p.Arg139Cys (R139C), in the nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) gene, has recently been identified as a strong risk factor for thiopurine-induced severe leukopenia in patients with IBD and ALL 6–9. Here, NUDT15 is linked to Decreased total leukocyte count.