An endogenous retrovirus (ERV) is a viral element in the genome that was derived from a retrovirus whose genome has been incorporated into the germ-line genome of some organism and is therefore copied with each reproduction of that organism. It is estimated that about 9 percent of the human genome originates from ERVs. In 2015 it was shown that proteins from an ERV are actively expressed in 3-day-old human embryos and appear to play a role in embryonal development and protect embryos from infection by other viruses.