A multicenter retrospective pharmacogenetic study in Japan that investigated the association of NUDT15 variants and haplotypes with adverse events in 2630 IBD patients (CD 1049, UC 1522, and BD 60) showed that NUDT15 p. Arg139Cys was associated with leukopenia and hair loss, but was not closely associated with delayed leukopenia. Here, NUDT15 is linked to inflammatory bowel disease.