HERVs are a class of multiple sclerosis associated retroviruses. Reacts with a replication competent Human Endogenous Retrovirus (HERVW) isolated from placenta. The synthetic peptide sequence used as immunogen is taken from the virus envelope polyprotein sequence. Retroviral envelope proteins mediate receptor recognition and membrane fusion during early infection. Endogenous envelope proteins may have kept, lost or modified their original function during evolution. The HERVW endogenous envelope protein has retained its original fusogenic properties and participates in trophoblast fusion during placenta morphogenesis.