其蛋白產品造成不可控的細胞生長（癌癥）的基因。正常細胞在細胞周期中分裂的過程是由很多基因控制的。癌基因有時產生于這些基因中的一個或多個突變。突變產生異常蛋白，不能行使正常功能調節細胞生長，或引發不可控細胞生長。癌基因也會存在于病毒，由病毒進入宿主細胞引發癌癥，這些往往是宿主細胞的蛋白突變型

A gene whose protein product causes uncontrolled cell growth (cancer). Normal cells divide under a very specific program called the cell cycle that is controlled by many genes. Oncogenes are sometimes produced as a result of a mutation in one or more of these genes. The mutation results in a malfunctional protein, which cannot perform its normal task to regulate cell growth or actually induces uncontrolled cell growth. Oncogenes are also found in viruses that introduce them into the host cells they infect to induce cancer; these are often mutant versions of proteins present in host cells.