Why viral stowaways are a baby’s best friend - health - 12 September 2006 - New Scientist: "Harmless viruses apparently stowed away for millions of years in the DNA of mammals have proved to be more than idle passengers.
New research in live sheep has demonstrated for the first time that they help embryos change shape, implant themselves in the womb and grow a placenta. The same almost certainly happens in other mammals, including humans, they say.
The findings provide new insights into how so-called endogenous retroviruses (ERVs) and mammals evolved together to the mutual advantage of both. ERVs typically account for 8% to 10% of the DNA in most mammals, including humans.
Far from being relics of infection dumped in the genetic equivalent of an attic, these viruses are turning out to play an active role in the way mammals develop and survive. Besides the role in pregnancy identified by the new research, they are already reckoned to help protect against harmful viruses, a role that could open up new avenues of medical treatment.