The available information regarding the role of the UGT1A1 genotype as a predictor of toxicity in cancer patients receiving irinotecan demands the performance of a randomized trial to ascertain whether genotype-adjusted dosages of the drug can help to establish safe and effective doses not only for patients with the UGT1A1*28 homozygous genotype but also for those with the most common UGT1A1 6/6 or 6/7 genotype. This evidence concerns the gene UGT1A1 and cancer.