MTHFR and colorectal carcinoma: In support of this, human cancer cell lines with the MTHFR 677 T allele show greater sensitivity to 5-FU compared with those with the C allele (Sohn et al, 2004) and this was confirmed in a study of 98 CRC patients treated with 5-FU-based chemotherapy (Etienne et al, 2004).