One way is with tumor suppressor genes, which usually function to inactivate or repress cell division. These genes or their products or both, must be inactivated sporadically for cell division of take place. If they are permanently inactivated or lost through mutation, uncontrolled cell division occurs. Another way cell division is regulated is by proto-oncogenes, which usually promotes cell division also. When the genes or their products or both are inactivated, cell division is stopped.
Oncogenes are the mutant form of proto-oncogenes.
An example of a transformation of a proto-oncogene to an oncogene is the p53 gene, which encodes a nuclear protein that acts as a transcription factor. The p53 gene is usually a tumor suppressor gene that controls passage of the cell from one phase of mitosis to another. The mutations in p53 gene are estimated to be associated with over half of all cancers. The most prevalent cause of death in cancer patients is metastasis, where cancer cells detach from the original tumor site and settle elsewhere in the body, to grow and divide producing another tumor. There are two kinds of tumors, benign and malignant.