Influenza A viruses lack the NS1 gene and replicate in interferon-deficient systems. The NS1 protein, a nonstructural protein of influenza A virus, plays a crucial role in inhibiting interferon-mediated antiviral responses. Using reverse genetics, a viable influenza A virus (delNS1) lacking the NS1 gene was generated. This virus, delNS1, was characterized and found to be dispensable for viral replication in interferon-deficient systems, suggesting that NS1 is a virally encoded inhibitor of interferon responses. The NS1 protein has been implicated in several regulatory functions during influenza virus infection, including inhibition of host mRNA polyadenylation, nuclear export of mRNAs, pre-mRNA splicing, and activation of the IFN-induced kinase PKR. These functions are critical for viral replication and pathogenesis. The delNS1 virus, which lacks the NS1 gene, was found to replicate poorly in MDCK cells and in 11-day-old embryonated eggs, but was able to replicate in Vero cells. When tested in STAT1-deficient mice, delNS1 virus was pathogenic, indicating that the absence of NS1 leads to altered growth properties. The delNS1 virus was found to stimulate the IFN response in 293 cells, suggesting that NS1 is involved in inhibiting the IFN response. The NS1 protein may inhibit IFN-mediated antiviral responses by binding to dsRNA, preventing the activation of PKR. The NS1 protein's amino-terminal RNA-binding domain is likely responsible for this function. The study highlights the importance of NS1 in influenza A virus pathogenesis and suggests that NS1 proteins in highly pathogenic strains may be more efficient in preventing IFN-mediated antiviral effects. The delNS1 virus, lacking the NS1 gene, may be a useful tool for studying IFN responses and could potentially be used as a therapeutic agent to stimulate an antiviral state against various infectious agents. The findings also suggest that NS1 is an auxiliary (virulence) factor in influenza A virus replication and that its absence leads to altered growth properties in different cell types and hosts.Influenza A viruses lack the NS1 gene and replicate in interferon-deficient systems. The NS1 protein, a nonstructural protein of influenza A virus, plays a crucial role in inhibiting interferon-mediated antiviral responses. Using reverse genetics, a viable influenza A virus (delNS1) lacking the NS1 gene was generated. This virus, delNS1, was characterized and found to be dispensable for viral replication in interferon-deficient systems, suggesting that NS1 is a virally encoded inhibitor of interferon responses. The NS1 protein has been implicated in several regulatory functions during influenza virus infection, including inhibition of host mRNA polyadenylation, nuclear export of mRNAs, pre-mRNA splicing, and activation of the IFN-induced kinase PKR. These functions are critical for viral replication and pathogenesis. The delNS1 virus, which lacks the NS1 gene, was found to replicate poorly in MDCK cells and in 11-day-old embryonated eggs, but was able to replicate in Vero cells. When tested in STAT1-deficient mice, delNS1 virus was pathogenic, indicating that the absence of NS1 leads to altered growth properties. The delNS1 virus was found to stimulate the IFN response in 293 cells, suggesting that NS1 is involved in inhibiting the IFN response. The NS1 protein may inhibit IFN-mediated antiviral responses by binding to dsRNA, preventing the activation of PKR. The NS1 protein's amino-terminal RNA-binding domain is likely responsible for this function. The study highlights the importance of NS1 in influenza A virus pathogenesis and suggests that NS1 proteins in highly pathogenic strains may be more efficient in preventing IFN-mediated antiviral effects. The delNS1 virus, lacking the NS1 gene, may be a useful tool for studying IFN responses and could potentially be used as a therapeutic agent to stimulate an antiviral state against various infectious agents. The findings also suggest that NS1 is an auxiliary (virulence) factor in influenza A virus replication and that its absence leads to altered growth properties in different cell types and hosts.