The enemy within: dormant retroviruses awaken http://www.nature.com/nm/journal/v16/n5/full/nm0510-517.html Mammalian genomes harbor regulatory elements from ancient retroviral infections. These retroviral remnants are normally silenced by DNA methylation—but this can change. Reactivation of one such element triggers the expression of a nearby oncogene during the development of Hodgkin's lymphoma (571–579). http://www.nature.com/nm/journal/v16/n5/full/nm.2129.html Derepression of an endogenous long terminal repeat activates the CSF1R proto-oncogene in human lymphoma Over 100 million years ago, retroviruses began their assault on mammals by infiltrating the mammalian genome. In defense, mammals developed the ability to recognize and silence transcription from these integrated retroviruses.