Recent genome-wide association studies (GWAS) have provided evidences of TPMT as the only monogenic trait that influences the TPMT phenotype, finding that only variants in TPMT gene are significantly associated with TPMT activity in ALL children at genome-wide level (n = 1,026; P = 8.6 × 10-61) and in a mixed-population (two cohorts of adult healthy volunteers and one of pediatric ALL patients, n = 1,212; P = 1.2 × 10-72)[23,24]. Here, TPMT is linked to acute lymphoblastic leukemia.