Normal cells have a mechanism to control the natural mutagenesis of mutagenized cells (i.e. tumor cells). Cancer cells typically loss the ability of natural apoptosis due to damage of normal cells or loss of tumor suppressor genes. Cancer gene therapy is basically injecting cancer suppressor genes to the cancer cells nucleus from outside into the body, and recover the normal environment for cell division and natural apoptosis, as a result, the cancer cells will naturally disappear from the body.