MLH1 and colorectal cancer: Currently, hypermethylation in promoter regions of some important tumor suppressor genes that cause gene expression inhibition has been identified in colorectal cancer cells (de Vallière et al., 2015), such as Cyclin-dependent kinase inhibitor 2A (CDKN2A) (Bihl et al., 2012), DNA mismatch repair protein Mlh1 (MLH1) (Cunningham et al., 1998), and Adenomatous polyposis coli protein (APC) (Liang et al., 2017).