The majority of colon cancer cases (at least eighty percent) are thought to be sporadic in nature. These occur in individuals with no identifiable hereditary disorder which predisposes to colon cancer, and are likely initiated by environmental carcinogens alluded to above leading to a series of accumulated genetic alterations or mutations which result in a malignant tumor. These types of mutations are called somatic mutations, meaning that they occur not in the germ cells of an organism, but in the cells of tissues which are already fully developed.

These somatic mutations lead to overproliferation of mucosal cells of the colon which then lead to aberrant crypt foci that can then become adenomas or polyps. Most polyps do not become cancerous. As will be seen below, a series of further somatic mutations is required before cancer develops. The complete sequence of mutations may take up to ten or twenty years before cancer develops.