NUDT15 and Decreased total leukocyte count: Yang et al. reported that a non-synonymous SNP in NUDT15 (also known as MTH2) that causes p.Arg139Cys (R139C) was very strongly associated with thiopurine-induced early leukopenia in Koreans [odds ratio (OR) 35.6; P = 4.88 × 10−94]; these authors also confirmed this association in patients with IBD who were of European ancestry (OR 9.50; P = 4.64 × 10−4), although it was very rare (minor allele frequency < 0.004) [70].