In a study of 131 rectal cancer patients, MTHFR 1298A>C and MTHFR diplotypes (for 677C>T and 1298A>C) were associated with 5-FU chemoradiation-related toxicity, with no impact on drug response [100]; however, it is unknown the extent to which radiation played a role in the risk of toxicity. Here, MTHFR is linked to rectal cancer.