This phase II multicenter clinical trial will use a genotype-guided dosing strategy for irinotecan to prospectively analyze efficacy in 100 metastatic colorectal cancer patients (mCRC) receiving FOLFIRI (5-fluorouracil (5-FU), leucovorin, irinotecan) plus bevacizumab. Irinotecan is detoxified and excreted primarily by glucuronidation in the liver via the isoenzyme uridine diphosphate glucuronosyl transferase (UGT1A1). Common variants in UGT1A1 alter the rate of glucuronidation and thus alter exposure to irinotecan.