Normally, cells divide to produce more cells only when your body needs them. Cancer occurs when the genes that control the healthy division of cells mutate or are damaged. As a result, cells grow out of control, disrupting normal body tissues and organ functions. Two types of genes are involved in most cancers.
Oncogenes promote normal cell division.
When an oncogene is damaged, it is like the accelerator of your car is glued to the floor. Abnormal cells divide recklessly. Tumor suppressor genes tell cells when to stop dividing. When tumor suppressor genes become disabled, they act like a faulty brake on a car, unable to stop runaway cell division. Normally, as new cells grow, old cells die to make way for them, a process called apoptosis.