A potential pathogenic significance of HERV-W Env/Syncytin-1 expression in MS can be inferred from data showing that Syncytin-1 has indirect cytotoxic effects on oligodendrocytes in vitro, and that expression of Syncytin-1 in murine models results in demyelination in vivo [18,22]. This evidence concerns the gene ERVW-1 and myeloid sarcoma.