Warning labels have always held a purpose in our society. When they're heeded, granted, they mark something as a force to be reckoned with, and avoided when necessary. They tell others that something dangerous is afoot, and that it is a risk they're best avoiding. So who knew that our own cells had such a system in place for suspicious patterns within our own DNA that enzymes should avoid replicating and transcribing, as scientists recently discovered that the enzyme methyltransferase, or Dnmt1, not only copies labels segments of DNA that are suitable, but will go out of its way to label other sequences of DNA with a methyl group that it finds dangerous, a job previously thought to be conducted only by the Dnmt 2 and 3 enzymes.
Article on virus sequences within our DNA: https://newsroom.uw.edu/news/genes-%E2%80%98fossil%E2%80%99-virus-human-dna-found-be-active