The anus is the opening to the outside of the body at the end of the bowel and rectum. There are several different types of anal cancer. About 9 out of 10 (90%) are squamous cell cancers, sometimes called epidermoid cancers. This group includes cancers called large cell keratinising, large cell non-keratinising (also called transitional) and basaloid cancers. Non keratinising and basaloid cancers are sometimes grouped together as 'cloacogenic' anal cancer. All these types of squamous cell anal cancer are treated in the same way. The other 1 out of 10 anal cancers (10%) are adenocarcinoma, small cell cancers, undifferentiated cancers and melanomas. This group is known as non-epidermoid cancers. They behave differently to squamous cell anal cancers, so the treatment is different.