ERVW-1 and germ cell tumor: To prevent undesirable formation of multinucleated syncytia in non-placental tissues, syncytin-1 expression is restricted to the placenta by several mechanisms which include epigenetic modifications of the 5' LTR regulatory region of ERVW-1, availability of specific transcription factors, and effective splicing of env mRNA that occurs exclusively in trophoblast and, aberrantly, in germ cell tumours [3–9].