UGT1A1 and neoplasm: The dose of the drug may modify the effect of the pharmacogenomic marker (e.g. irinotecan dose modifies the effect of UDP glucuronosyltransferase 1A1 genotype on irinotecan toxicity but not tumor response [28], [29]) and thus if the RCT and association studies have different irinotecan doses this will bias the subgroup treatment toxicity estimates.