UGT1A1 and dihydropyrimidine dehydrogenase deficiency: 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.