Because many patients with wild-type TPMT and NUDT15 clinically experience myelosuppression by thiopurine, many cases of thiopurine-induced leukopenia cannot be explained by genetic variants in TPMT or NUDT15. Kim et al. have reported that fat mass and an obesity-associated gene (FTO) p.Ala134Thr (rs79206939) are associated with thiopurine-induced leukopenia according to high-throughput targeted sequencing for GWAS and fine mapping of 1,098 patients with IBD in Korea and Japan. Here, NUDT15 is linked to inflammatory bowel disease.