The phosphorylated and unphosphorylated forms of STAT1 were detected using anti-STAT1 (Y701) antibody and anti-STAT1 (BD Bioscience). An antibody directed against Y690 of STAT2 was used for detection of the phosphorylated form of STAT2 (Upstate). Antibodies to detect influenza viral proteins were purchased from Serotec (NP, M1), Santa Cruz (PB1, PB2). The anti-PA antibody was kindly provided by J. Ortin (Madrid/ Spain). A monoclonal antibody directed against the viral NS1 was generated at the IMV, Muenster, Germany [70] . A monoclonal anti-Myc-tag antibody to detect Myc-M1 was kindly provided by Viktor Wixler. IMV, Muenster, Germany. All secondary antibodies were purchased from Amersham and diluted 1:2500 in TBS-T. Secondary antibodies were incubated for a minimum of 60 minutes at room temperature.