Viruses are known to use the genetic machinery of the human cells they invade to make copies of themselves. As part of the process, viruses leave behind remnants throughout the genetic material (genomes) of humans. The virus-like insertions, called “transposable elements,” are snippets of genetic material even simpler than viruses that also use host cell machinery to replicate.

Nearly all these inserted elements have been silenced by our cells’ defense mechanisms over time, but a few, nicknamed “jumping genes,” can still move around the human genome like viruses. Just one, called long interspersed nuclear element 1 (LINE-1), can still move by itself.

As an element type that behaves like the retrovirus HIV, the LINE-1 “retrotransposon” is first copied into a molecule of RNA, the genetic material that partners with DNA, and then the RNA LINE-1 copy is converted back into DNA in a new place in the genome.