Approximately 70% of all human genes are expressed in colorectal cancer, with just over 1% of having increased expression in colorectal cancer compared to other forms of cancer.  Some genes are oncogenes : they are overexpressed in colorectal cancer. For example, genes encoding the proteins KRAS , RAF , and PI3K , which normally stimulate the cell to divide in response to growth factors, can acquire mutations that result in over-activation of cell proliferation. The chronological order of mutations is sometimes important. If a previous APC mutation occurred, a primary KRAS mutation often progresses to cancer rather than a self-limiting hyperplastic or borderline lesion.  PTEN , a tumor suppressor, normally inhibits PI3K, but can sometimes become mutated and deactivated.