All of these genes have four things in common. Each one derives from the envelope gene of a retrovirus that inserted itself in the mammal genome. Each one expresses itself as a protein suffusing the placenta. Each one causes cell-to-cell fusion (at least in lab cultures), suggesting that it can create that special fused-cell protoplasmic layer, which helps mediate between placenta and fetus, letting nutrients and gases seep in from the mother, letting wastes seep out. And each is an ancient gene, preserved for many millions of years in functional form (against the disarray of random mutations)
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