This is justified by the facts that the retroviral envelope protein from HERV-W endogenous retrovirus family (i) has been regularly detected in MS brain lesions [20], [25], [26], [32], (ii) can be abnormally expressed upon HERV-W activation by certain environmental viruses in permissive cells [48], [49] and (iii) is now shown to trigger in vivo the pathogenic features characterizing MS lesions. Here, ERVW-1 is linked to myeloid sarcoma.