Light micrograph of a section of human tissue showing an example of mouth cancer. Mouth cancer, also known as oral cancer, is where a tumour develops in a part of the mouth. The cancer may begin on the surface of the tongue, the inside of the cheeks, the roof of the mouth, or the lips or gums. Tumours can also develop in the glands that produce saliva, the tonsils and part of the throat connecting the mouth to the windpipe (pharynx). However, these are less common.