KRAS and colorectal cancer: Early colorectal cancer and colorectal precancerous lesions are associated with three major genetic mechanisms: first, chromosomal instability caused by mutations in APC, KRAS, and TP53; second, microsatellite instability resulting from the functional loss of mismatch repair genes; and third, DNA methylation, an epigenetic change that leads to promoter hypermethylation and subsequent gene transcription suppression (15).