HERVS BUILD PLACENTA. “One could postulate that HERV elements may have a beneficial role in human development. Since retroviral envelope proteins have fusogenic properties, it was assumed that HERV-encoded Env proteins may play a role in the formation of the syncytiotrophoblast layer of the placenta. The hypothesis that Env proteins are involved in syncytiotrophoblast formation has been recently confirmed.”
http://jvi.asm.org/content/79/1/341.full