A genetic variation in the gene for the protein dihydrofolate reductase (DHFR) that is necessary to utilize folic acid (a synthetic form of the B vitamin folate found in supplements and fortified food), increases the risk for breast cancer in multivitamin users and, when present in mothers who used folic acid supplements during pregnancy, increases the risk for cancer of the eye of their children. The aim of the proposed research is to understand how a common genetic variation in the gene for DHFR affects the function of this protein and the ability of the body to use folic acid.