TP53 and colorectal cancer: Although smoking is not associated with the risk for colorectal cancer with KRAS oncogene mutations, it may be an early event in the development of colorectal cancers that arise through other underlying genetic pathways, such as mutations in the adenomatous polyposis coli (APC) tumor suppressor gene, P53 over-expression or absence of MLH1 protein expression [25, 26].