To explore the possibility that IL-2 may play a role in the pathology of MS in conjunction with viral infection, we constructed a recombinant herpes simplex virus type 1 (HSV-1) that expresses murine IL-2 constitutively [13] as well as a panel of control recombinant viruses that express murine IL-4, interferon (IFN)-γ, IL-12p35, or IL-12p40 continuously [14], [15], [16]. This evidence concerns the gene IL2 and myeloid sarcoma.