The remaining question was, whether the elevated IFN-induced gene response in knock out cells might also affect propagation of influenza A viruses. Thus, both wild type and knock out cells were infected with PR8 ( Figure 8A ) or the strain A/Victoria/3/75 (H3N2) ( Figure 8B ). Virus titers were assessed at different time points post infection. Progeny virus titers from SOCS-3 knock out cells were significantly reduced compared to titers from infected wild type cells. To independently confirm these results and to verify that the observed effects are really due to the lack of SOCS-3, we used an siRNA approach to specifically knock down SOCS-3 mRNA in A549 cells. Cells were transfected with 150 nM siRNA for 48 h and SOCS-3 protein levels were compared to control transfected samples ( Figure 8C, right) . Subsequently, cells were infected and progeny virus titers were determined by plaque assay (Figure 8C, left) . Similar to the results gained from infected knock out cells, knock down of SOCS-3 resulted in decreased virus titers.