Most normal cells grow, reproduce and die in response to signals inside and outside the body. If these processes occur in a balanced and orderly way, the body remains healthy and functions normally. Problems can begin when normal cells mutate, or change, into cancer cells. A normal cell can become a cancer cell for no apparent reason or because of repeated or heavy exposure to a carcinogenic, or cancer-causing, substance such as tobacco or alcohol. Both the behavior and appearance of cancer cells differ from normal cells.
Mutation occurs in the cell's DNA, or genetic material.
DNA controls the cell's behavior and appearance. Once DNA has mutated, the cell is different from the healthy cells nearby. The mutated cell appears different and functions in other ways from normal cells. The mutated cell divides into two new mutated cells that divide into four mutated cells. This process continues until the mutated cells form a mass called a tumor.