ERVW-1 and infection: Along this line, it has been proposed that the HERV envelope proteins could play a role in several processes including i) protection against infection by present-day retroviruses through receptor interference [7], ii) protection of the fetus against the maternal immune system via an immunosuppressive domain located in the envelope transmembrane (TM) subunit [8,9], and iii) placenta morphogenesis through fusogenic effects allowing differentiation of cytotrophoblastic cells into the syncytiotrophoblast [10-12].