Several reporter assays have been described to study and quantify IAV transcription/replication in vivo. These reporter systems usually consist of a reporter protein-encoding cDNA, flanked by 39 and 59 UTRs, inserted in an antisense orientation between a PolI promoter and a transcription terminator or ribozyme sequence. After introduction of the reporter construct into a cell, reporter gene expression is induced by co-transfection of plasmids encoding NP, PA, PB1 and PB2 (transfection assay) or by subsequent infection with a helper IAV (infection assay). Such reporter assays are very helpful to quantify virus replication or virus production, and to assess the antiviral activity of compounds including antibodies [20] [21] [22] . These assays have also been used for the mutational analysis of IAV promoter elements in vivo [12, 23, 24] .