Cancer of the colon is one of the most common causes of death from cancer, ranking just behind lung cancer, among the types of cancer that affect men and women. Cancer can begin anywhere in the colon. The right colon begins in the right lower part of the abdomen. It ascends to the right upper part, across to the left upper part (in which it meets the left colon), and descends to the anus (the left colon). Most cancers of the colon begin as polyps—small, slow-growing, mushroom-like growths on the inner surface of the colon. Mutations in the genes that control cell division lead to more rapid growth and eventually invasion into the wall of the colon and beyond. The purpose of colon cancer screening is to detect and remove growths before they invade and spread. Experts recommend several effective screening methods. One is to inspect the entire colon through a flexible tube (colonoscopy), which is the most accurate. Because colonoscopy is the most expensive, inconvenient, and riskiest test, it is important to know whether colonoscopy reduces the chance of dying of colon cancer.