Breast Cancer has been associated with polymorphisms in genes candidate to be disease modifiers, such as CYP19 (Family 19 of the Cytochrome P450), GSTP1 (Glutathione S-Transferase Protein), TP53 (Tumor Protein 53), P21 (Protein 21) [19], MTHFR (Methylenetetrahydrofolate Reductase) and TYMS (Thymidylate Synthase) [20] and, which are involved in events such as synthesis, methylation and DNA damage repair, and cellular activation in carcinogen metabolism and the metabolism of anticancer drugs. The gene discussed is TYMS; the disease is breast carcinoma.