The investigators propose to study the effect of these two polymorphic enzymes on capecitabine's toxicity in adjuvant colon cancer patients. It is anticipated that patients homozygous for 2R/2R will have higher rates of overall toxicity, diarrhea, neutropenia and mucositis than 3R/3R homozygotes. For MTHFR, we anticipate that TT homozygotes will have higher rates overall toxicity, diarrhea, neutropenia, and mucositis than CC homozygotes. The effect of MTHFR polymorphism on capecitabine's toxicity will be examined controlling for thymidylate synthase genotype.