While adjustments in the dose of irinotecan seem to be sufficient to decrease the risk of severe toxicity even in patients with homozygous UGT1A polymorphism [32], treatment with a fluoropyrimidine in patients with severe DPD deficiency is associated with a prohibitively high risk of severe toxicity and is not recommended [33]. This evidence concerns the gene UGT1A1 and dihydropyrimidine dehydrogenase deficiency.