DNMT1 and colorectal carcinoma: Colorectal carcinoma cell line HCT116 and its derivative cell line DKO, which bears homozygous deletions of both DNMT3b and DNMT1, are widely used models to study target genes of DNA methylation-mediated silencing, especially for tumor suppressor genes (TSGs)[32-36] and miRNAs[24,37].