Colorectal cancer cells. Coloured scanning electron micrograph (SEM) of a culture of a Caco-2 adenocarcinoma cancer cells from the human colon (large intestine). Adenocarcinomas of the colon and rectum make up 95 percent of all colorectal cancer cases. In the gastrointestinal tract, rectal and colon adenocarcinomas develop in the cells of the lining inside the large intestine. These adenocarcinomas typically start as a growth of tissue called a polyp. A particular type of polyp called an adenoma may develop into cancer. Polyps are often removed during a routine colonoscopy before they may turn cancerous. Symptoms include rectal bleeding and abdominal pain. Treatment is with surgery to remove the affected area. Colon cancer is one of the most common cancers in the Western world. Magnification: x 4000 when printed 10 centimetres wide.