Defending the genome from viral integration

 Small, mobile sequences of DNA left over from viruses, called transposons or "jumping genes" because of their ability to move around the genome, pose a significant threat to the genetic integrity and stability of an organism. Considered genetic parasites, these transposable elements are believed to comprise as much as 50 percent of the human genome. Because of the damage transposons can do to an organism's DNA, an immune-like response has evolved to turn off, or silence, the transposons.