MLH1 and colorectal cancer: Given that colorectal cancer patients with DNA mismatch repair deficiency do not benefit from 5-FU based chemotherapy, we used a pair of isogenic cell lines, HCT116 (MMR-deficient, due to hypermethylation of MLH1 gene) and HCT116+ch3 (MMR-proficient, due to stable transfer of chromosome 3 bearing wild type copy of the MLH1 gene).