IRF3 and infection: The phosphorylated form of IRF3 was barely detectable following infection with the influenza virus strains A/WSN/33 (H1N1) (WSN) and A/PR/8/34 (H1N1) (PR8) in untreated cells, while A/Udorn/72 (H3N2) (Ud) induced small amounts of IRF3 phosphorylation (Fig. 1A and B).