by Jamie Henzy — Ver­te­brate genomes are teem­ing with se­quences from retro­viruses that have in­fected the cells of the germ line over the course of evo­lu­tion. Known as en­doge­nous retro­viruses (ERVs), the vast ma­jor­ity of these once fierce foes have been de­fanged by mu­ta­tions that make them in­ca­pable of ex­press­ing vi­ral pro­teins, let alone in­fec­tious par­ti­cles.