Fig. (1).

There are three genetic domains, flanked by the regulatory LTRs, or long terminal repeats. The gag domain codes for matrix and core shell proteins, the pol codes for reverse transcriptase, protease, ribonuclease and integrase, and the env codes for the surface and trans-membrane glycoproteins. The env of a HERV-W is also responsible for syncytiotrophoblast cell fusion in the human placenta –see Fig. (4).