Proto-oncogene: A normal gene which, when altered by mutation, becomes an oncogene that can contribute to cancer. Proto-oncogenes may have many different functions in the cell. Some proto-oncogenes provide signals that lead to cell division. Other proto-oncogenes regulate programmed cell death (apoptosis).
The defective versions of proto-oncogenes, known as oncogenes, can cause a cell to divide in an unregulated manner. This growth can occur in the absence of normal growth signals such as those provided by growth factors. A key feature of oncogene activity is that a single altered copy leads to unregulated growth.