A polymorphism in methylene tetrahydrofolate reductase's (MTHFR) gene may also determine a patient's risk for capecitabine toxicity. A polymorphism in MTHFR exists at position 677, C to T producing a thermolabile and rapidly degraded enzyme. TT homozygotes have increased levels of methylene tetrahydrofolate. Methylene tetrahydrofolate stabilizes binding of 5FU to thymidylate synthase and the complex of TS, 5FU, and methylene tetrahydrofolate is referred to as the ternary complex. I hypothesize that increased stabilization of TS and 5FU due to increased amounts of methylene tetrahydrofolate would lead to increased capecitabine toxicity and efficacy. No study has examined if TT homozygotes have an increased response rate to fluoropyrimidines or increased toxicity.