The present invention relates, in general, to a screening method for identifying novel viral proteins with interferon antagonizing function using a transfection-based assay, and the use of such proteins in isolating various types of attenuated viruses for the development of vaccine and pharmaceutical formulations. The invention also relates to the use of viral interferon antagonists in screening assays to identify potential anti-viral agents. The invention further relates to protocols utilizing interferon antagonists, e.g., NS1, to enhance gene therapy or DNA vaccination based on their ability to increase gene expression.