A study from Madrid showed that the authors could not confirm whether the choice of AZA or a mercaptopurine dose based on TPMT activity prevented leucopenia in patients with IBD.12 Reports have suggested that variation in TPMT activity accounted for no >10% of overall thiopurine toxicity and approximately one-third of cases of leucopenia.13 This evidence concerns the gene TPMT and inflammatory bowel disease.