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Illustration of how a normal cell is converted to a cancer cell, when an oncogene becomes activated

An oncogene is a gene that has the potential to cause cancer. Or, equally good, an oncogene is a gene which brings about uncontrolled cell division.

Oncogenes in tumor cells are often mutated or expressed at high levels.

Most normal cells undergo a programmed form of death (apoptosis). Activated oncogenes can cause those cells designated for apoptosis to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target the proteins coded by oncogenes.