1) A gene that normally directs cell growth. If altered, an oncogene can promote or allow the uncontrolled growth of cancer. Alterations can be inherited or caused by an environmental exposure to carcinogens.

2) A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.

3) A gene having the potential to cause a normal cell to become cancerous.

4) An oncogene is a mutated gene that contributes to the development of a cancer. In their normal, unmutated state, onocgenes are called proto-oncogenes, and they play roles in the regulation of cell division. Some oncogenes work like putting your foot down on the accelerator of a car, pushing a cell to divide. Other oncogenes work like removing your foot from the brake while parked on a hill, also causing the cell to divide.