- Correlate the presence of the C677T polymorphism of the methylenetetrahydrolate reductase gene, the presence of a polymorphism in the enhancer region of the thymidylate synthase (TS) gene promoter (2R and 3R tandem repeats), the presence of a polymorphism within one of those repeats, and the presence of a functional polymorphism in the 3'-untranslated region with toxicity in patients treated with this drug.