其蛋白产品造成不可控的细胞生长（癌症）的基因。正常细胞在细胞周期中分裂的过程是由很多基因控制的。癌基因有时产生于这些基因中的一个或多个突变。突变产生异常蛋白，不能行使正常功能调节细胞生长，或引发不可控细胞生长。癌基因也会存在于病毒，由病毒进入宿主细胞引发癌症，这些往往是宿主细胞的蛋白突变型
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.