Colorectal cancer is cancer of the colon (large intestine) or rectum. Most colorectal cancer develops from small growths called polyps. Adenomas, which are the most common type of polyp, are the only type that can become cancer. Doctors use flexible tubular instruments to examine the colon (colonoscopy or sigmoidoscopy) and find polyps. To distinguish adenomas from other types of polyps, they remove or take pieces of them (biopsies) and send the tissue for special examination (histologic examination).