The purpose of this research study is to determine if dosing irinotecan based on genotype is effective and safe for patients with colon cancer. Patient genotype will be determined from a small sample of blood and a laboratory test or "assay" performed at UNC Laboratories. For the purpose of this study, this assay is new and considered to be "investigational". This means that the genotype assay used in this study has not yet been approved by the FDA for determining irinotecan dose levels in patients with colon cancer.