Regarding toxicity aspects, current evidence highlights the association between dihydropyrimidine dehydrogenase deficiency and an increased risk of fluoropyrimidine-related toxicity.25 The additive value of combined UGT1A1/DPYD genotype analysis could improve the safety of irinotecan plus fluoropyrimidine combinations and should be considered in further studies using these drugs. This evidence concerns the gene DPYD and dihydropyrimidine dehydrogenase deficiency.