Develops when cells in your body begin to grow out of control. Normal cells grow, divide, and die naturally. Instead of dying, cancer cells continue to grow and form new abnormal cells. Cancer cells often travel to other body parts where they grow and replace normal tissue. This process is called metastasis. Cancer cells develop because of damage to DNA. DNA is in every cell and directs all its activities. When DNA becomes damaged, the body is usually able to repair it. In cancer cells, the damage is not repaired. People can inherit damaged DNA, which accounts for inherited cancers. Many times, DNA becomes damaged by exposure to something in the environment, like smoking. Many cancers have no known cause.