The central hypothesis of this trial is that increasing the irinotecan dose in *1/*28 and *1/*1 genotypes will increase the overall benefit of FOLFIRI for patients with mCRC as these two groups are likely under-dosed with the current dosing regimen. Eligible patients will be genotyped for UGT1A1 and assigned into 1 of 3 different dosing groups, based on their relative rate of metabolism. The primary objective of this trial is to estimate progression-free survival (PFS), and secondary objectives include characterization of toxicity and objective response rate (OR; complete response (CR) + partial response (PR)).