ISG20 and infection: Therefore, we investigated expression of IL-15 and ISGs, such as APOBEC3G, ISG15, ISG20, MX1, MX2, and RSAD2 (Fig. 1a), after infection with R5-tropic, non-opsonized (HIV) and complement-opsonized (HIV-C) HIV-1 strains (BaL and YU-2).