MTHFR and breast cancer: The MTHFR C677T functional polymorphism, and the metabolism of folate, methionine, and homocysteine have been extensively studied in relation to etiologically complex chronic diseases, and a relationship between MTHFR C677T and age at onset have been observed in for example, coronary artery disease, breast cancer, and Parkinson disease [Mager et al., 2005; Lima et al., 2007; Lin et al., 2007].