MX1 and infection: To determine if the decrease in IFN transcription observed in cells infected with IVb NV null virus compared to WT virus was due to a decrease in the antiviral response to infection or an increase in the shutdown of the host antiviral response by IVb NV null VHSV, we assessed the impact of VHSV IVb infection on MX1 promoter/luciferase activity with and without exogenous IFN treatment.