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By Mayo Clinic staffMouth cancer occurs when cells on your lips or in your mouth develop changes (mutations) in their DNA. These mutations allow cancer cells to grow and divide when healthy cells would die. The accumulating mouth cancer cells can form a tumor. With time they may spread to other areas of the mouth and on to other areas of the head and neck or other parts of the body.
Mouth cancers most commonly begin in the flat, thin cells (squamous cells) that line your lips and the inside of your mouth. Most oral cancers are squamous cell carcinomas.
It's not clear what causes the mutations in squamous cells that lead to mouth cancer. But doctors have identified factors that may increase the risk of mouth cancer.