Syncytins are proteins derived from retroviral genes incorporated into the genomes of mammalian ancestors. Some syncytins help mediate cell fusion and development in the placenta.
The mammalian placenta depends on a protein that came from a virus. An ancient retroviral sequence was repurposed to make syncytin, which helps cells fuse at the boundary between mother and fetus. Placental mammals may partly owe their birth system to an old viral infection.