In contrast, infection of MEFs with knock-out of the IFN regulatory factor (IRF)-3 and -7 genes (IRF-3/7−/−; cannot effectively produce IFN-α/β but can respond to exogenous IFN) demonstrated no appreciable difference in replication efficiency between FL-IRAΔCS3 and FLSDX viruses. This evidence concerns the gene IFNA1 and infection.