Anal cancer is a rare form of cancer that occurs when cells in the anus begin to grow out of control. The anus is the opening at the lower end of the intestines where stool, or feces, passes out of the body as a bowel movement. There are different types of cells inside the anal canal. Most anal cancers start from cells in the inner lining of the anal canal, known as the mucosa. In the United States, most (nearly 9 out of 10 cases) anal cancers are squamous cell cancer. These tumors start in the squamous cells that line the surface of the anal canal and have grown into the deeper layers of the lining. Other types of anal cancers begin in different types of cells located in the anus.