The binding of IFNa/b to the type I IFN receptor results in the autophosphorylation and activation of JAK (Janus activated kinase) 1 and tyrosine kinase 2, which are both members of the JAK family and associated with the receptor (reviewed in [18] ). The activated JAKs phosphorylate specific tyrosines of STAT1 (signal transducer and activator of transcription 1) and STAT2. Upon phosphorylation, STAT1 and STAT2 form heterodimers and, in association with other factors, translocate to the nucleus to bind to ISREs to initiate transcription of ISGs. Binding of IFNc to its receptor similarly induces phosphorylation of STAT1 which then forms homodimers that translocate to the nucleus to activate the transcription of genes with GAS elements. Targeting the activation of STAT proteins or their transfer to the nucleus are efficient ways to block innate immunity that are used by several different viruses (reviewed in [13] ). To investigate whether NSDV/GV are inhibiting IFN action through an effect on STAT phosphorylation, we infected Vero cells with NSDV or GV for 16 hours and then stimulated the infected cells with IFNa or IFNc. Samples from those cells were subjected to immunoblot analysis to check the phosphorylation status of STAT1 and STAT2 ( fig. 4 ). In uninfected cells IFNa treatment induced phosphorylation of STAT1 and 2 ( fig. 4a ). An almost complete block of STAT1 phosphorylation was observed in NSDV infected cells, especially at later time points. The GV isolate also inhibited STAT1 phosphorylation, though was clearly less effective. This reduced effectiveness of the GV isolate corresponds to a reduced ability to block IFN action in the reporter gene assay ( fig. 3 ), and may be due to the slightly slower growth of this isolate in cell culture; at later time points of infection (18hpi) the GV isolate was The GV L protein inhibits transcription from the IFNb promoter So far the viral RNA-dependent RNA polymerase (RdRP) from CCHFV and DUGV are the only nairoviral proteins known to interfere with the host innate immune response [24] . We wanted to identify the NSDV protein(s) that is/are responsible for the inhibitory effects on IFN induction and action in infected cells. For that reason we made viral protein expression plasmids that were derived from the GV isolate as it is far less tissue culture adapted than the NSDV isolate. We cloned the open reading frame (ORF) of the S and M segments of GV into a mammalian expression vector under the control of a CMV promoter and with a V5-tag in frame at the C-terminus of each protein. For the M segment, which is translated into a polyprotein that produces at least two glycoproteins, this produces a V5 tag at the C terminaus of Gc, the most distal of the glycoproteins produced from this segment. We were unable to clone the complete ORF of GV (or NSDV) L into a plasmid due to recombination events that took place in Escherichia coli, even in strains such as STBL2, SURE2, ABLE K, XL-10 and MDS42 which are engineered to support unstable DNA. We mapped the toxic/unstable sequence to a region of approximately 1 kb found roughly in the middle of the L ORF. We were able to construct plasmids containing either half of this sequence but not the whole piece, and were therefore able to prepare expression constructs encoding the amino-and carboxy-terminal parts of GV L protein (aa L1-1757 and aa L1749-3391 respectively), thereby covering the whole protein with a short overlap between the constructs. In addition we cloned two shorter fragments of the amino terminal part of the viral polymerase that contained the OTU domain (aa L1-169) or the OTU domain and the following zinc-finger domain (aa L1-667). We transfected different amounts of these expression plasmids to approximately equalize the expression levels of the individual viral proteins in the reporter gene assays. In fig. 5a typical protein expression levels are shown that were used in our studies. Only the expression of the glycoprotein Gc which had the carboxy-terminal V5 tag could be analysed. The level of Gn could not be checked due to the lack of glycoprotein-specific antibodies for NSDV. However we assumed that the expression levels of the two glycoproteins are similar as they are expressed as a single polyprotein.