Cancer is caused by a gene mutation. There are mutations that occur in normal genes called proto-oncogenes. Proto oncogenes regulate what type of cell it is and how often it divides. These genes, when mutated, become oncogenes and the cell can divide out of control, forming a tumor. Another cause of cancer is when there is a mutation in a tumor suppressor gene. These are genes that correct mistakes in DNA, regulate cell division and tell cells when to die. A mutation here can also result in abnormal cell division and cancer. Other types of cancer occur in somatic cells that are exposed to carcinogens during a person's life. The longer you live, the greater the chance that healthy cells can become mutated and cancer can occur.