A gene that has the potential to cause cancer (not to be confused with a proto-oncogene, which is a potential oncogene - that is, it can be easily converted to an oncogene by mutation). Many oncogenes affect the rate of division of the cell, such as cyclins and are necessary to the normal function of the cell. Naturally, these oncogenes have to be activated into thier cancerous tendancies. This can be caused by viruses, chromosomal rearrangement, amplification of the gene (to produce many copies) or point mutation.