不过人体是一个森严的体系，决不允许有细胞叛变。每个细胞中同时也存在抑癌基因，抑癌基因可以抑制细胞周期，阻止细胞数目增多。如果有必要，它可以直接促使细胞死亡。
然而人体内总有意外发生，当原癌基因激活程度过强，或者当抑癌基因的两个等位基因同时突变而导致失活时，就有可能导致癌症的发生。
一般来说，癌症的发生，往往需要多种原癌基因或者抑癌基因发生突变，所以癌症的遗传风险其实并不是主要的，相反多达90%以上的癌症，都受到后天因素的影响。
参考资料
[1] Anand P, Kunnumakkara AB, Sundaram C, Harikumar KB, Tharakan ST, Lai OS, Sung B, Aggarwal BB. Cancer is a preventable disease that requires major lifestyle changes. Pharm Res. 2008 Sep;25(9):2097-116.
[2] Hanahan D , Weinberg R A . Hallmarks of cancer: The next generation[J]. Cell, 2011(5):144.
[3] GBD 2015 Disease and Injury Incidence and Prevalence Collaborators. Global, regional, and national incidence, prevalence, and years lived with disability for 310 diseases and injuries, 1990-2015: a systematic analysis for the Global Burden of Disease Study 2015. Lancet. 2016 Oct 8;388(10053):1545-1602.
[4] Schnekenburger M, Diederich M. Epigenetics Offer New Horizons for Colorectal Cancer Prevention. Curr Colorectal Cancer Rep. 2012 Mar;8(1):66-81.
[5] Baldassarre G, Battista S, Belletti B, Thakur S, Pentimalli F, Trapasso F, Fedele M, Pierantoni G, Croce CM, Fusco A. Negative regulation of BRCA1 gene expression by HMGA1 proteins accounts for the reduced BRCA1 protein levels in sporadic breast carcinoma. Mol Cell Biol. 2003 Apr;23(7):2225-38.
[6] Kanwal R, Gupta S. Epigenetic modifications in cancer. Clin Genet. 2012 Apr;81(4):303-11.
