A study by Cohen et al. [7] is the first that reported a link between the MTHFR genotype and tumor response to 5-FU-based chemotherapy, among which of 43 metastatic colorectal cancer patients receiving exclusive 5-FU therapy, all five 677TT patients responded to the treatment, whereas the response rate was approximately 50 % in 677CC patients. The gene discussed is MTHFR; the disease is metastatic colorectal cancer.