CDKN2A and colorectal cancer: Moreover, DNA methylation may facilitate the mutation of TSGs, such as mutated p53 [20], or contribute directly to tumorigenesis, such as the hypermethylation in APC-1A, CDKN2 and RASSF1A genes in colorectal cancer [21], or the hypermethylation in MTHFR gene in lung cancer [22].