When activation of the IFN induction cascade by these viruses was examined by flow cytometry, infection with Ud-Δ99/R38A and PR8-ΔNS1 resulted in a higher number of GFP-positive infected cells (Fig. 1d, e) than seen for Ud wt or PR8 wt (Fig. 1a), due to alleviation of the NS1-mediated inhibition of IFN expression that exists during wt virus infections. The gene discussed is IFNA1; the disease is infection.