ISG15 and infection: Remarkably, following MVA infection in DF-1 STING Knock-out (DSK) cells, we observed reduced expression of IFNα and IFNβ genes along with Interferon-stimulated gene 15 (ISG15), a major protein induced by type I interferons (20), interferon-induced transmembrane protein 3 (IFITM3), which interferes with vaccinia virus entry into the cytoplasm (21), and other interferon-stimulated genes (ISGs).