NUDT15 and Decreased total leukocyte count: The first epoch-making discovery was reported by Yang et al., who specifically conducted a genome-wide association study (GWAS) in Korean CD to reveal that a non-synonymous SNP, p.Arg139Cys (R139C), in nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) is very strongly associated with thiopurine-induced severe leukopenia [11].