<P> Most normal cells will undergo a programmed form of rapid cell death ( apoptosis ) when critical functions are altered and malfunctioning . 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 normal genes promoting cellular growth , through mutation , are up - regulated , ( gain of function mutation ) they will predispose the cell to cancer and are thus termed oncogenes . Usually multiple oncogenes , along with mutated apoptotic and / or tumor suppressor genes will all act in concert to cause cancer . Since the 1970s , dozens of oncogenes have been identified in human cancer . Many cancer drugs target the proteins encoded by oncogenes . </P>
a proto-oncogene is best defined as what type of gene
normal genes involved in cell growth and proliferation or inhibition of apoptosis