In vitro, Epstein-Barr virus (EBV) activates HERV-W/MSRV/syncytin-1 in cells derived from blood and brain [33] and antigens from other herpesviruses as HSV-1, HHV-6, and VZV are able to induce higher RT activity in peripheral lymphocytes from MS patients compared to controls [34]. This evidence concerns the gene ERVW-1 and myeloid sarcoma.