The type I interferon (IFN) system is one of the most powerful innate defenses against viral pathogens. Most RNA viruses are sensitive to the action of type I IFN. Therefore, these pathogens have evolved strategies to evade this response. For example, influenza viruses express a viral protein, the non-structural protein 1 (NS1), that suppresses production of IFNb by lowering cellular sensitivity to viral nucleic acid as a pathogen pattern.