A schematic overview of the dual luciferase reporter constructs and the assays is shown in Figure 1 . The firefly and Gaussia luciferase genes, in this example flanked by 39 and 59 UTRs of the NP segment (referred to as FNP and GNP, respectively), are inserted in an antisense orientation between a PolI promoter and a ribozyme sequence. Cells are transfected with either one or both reporter constructs (single or co-transfection). Luciferase expression is induced by expression of viral RNA polymerases and NP either by co-transfection of expression plasmids (transfection assay) or by virus infection (infection assay). The expression levels of the firefly and Gaussia luciferase reporter constructs are determined consecutively using a single tube, dual luciferase assay system.