Although the total number of TPMT variants that have been discovered is more than 20 (TPMT*2–24) [4, 17], the most common ones are TPMT*2 (found in an 8-year-old girl treated for ALL who developed severe haematological toxicity from conventional oral doses of 6-MP [4]) and TPMT*3A, both of which cause amino acid changes leading to breakdown of TPMT protein and thus decreased activity [4, 18] (see also Table 1). Here, TPMT is linked to acute lymphoblastic leukemia.