An oncogene is a gene responsible for the normal growth and differentiation of cells but which is also capable of causing the transformation of normal cells into cancer cells. Oncogenes are sections of DNA found in all cells of the body; however, of the full human complement of 50,000 genes, fewer than 100 are probably oncogenes.

Proto-oncogenes are present in all normal cells and in some viruses. Oncogenes arise from the mutation of proto-oncogenes. Some of these mutations are associated with various cancers. The oncogene concept led to the development of the onco-mouse, a laboratory animal that has one of these rogue genes implanted into its cells; it is used in testing anti-cancer treatments.