oncogene

thumb|300px|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. In tumor cells, these genes are often mutated, or expressed at high levels.

Most normal cells undergo a preprogrammed rapid cell death (apoptosis) if critical functions are altered and then malfunction. Activated oncogenes can cause those cells designated for apoptosis to survive and proliferate instead. Most oncogenes began as proto-oncogenes, normal genes involved in cell growth and proliferation or inhibition of apoptosis. If, through mutation, normal genes promoting cellular growth are up-regulated (gain-of-function mutation), they predispose the cell to cancer and are termed oncogenes. Usually, multiple oncogenes, along with mutated apoptotic or tumor suppressor genes, act in concert to cause cancer. Oncogenes are a physically and functionally diverse set of genes, and as a result, their protein products have pleiotropic effects on a variety of intricate regulatory cascades within the cell.