Interestingly, we found that BTV1-DNS4 replicated as efficiently as wild type BTV-1, even in cells treated with IFN. However, the NS4 of BTV-1 appears to possess the same biological properties of the NS4 of BTV-8. Indeed, a BTV-8 reassortant containing the entire segment 9 of BTV-1 (BTV-8/ 1S9) or a recombinant BTV-8 expressing an NS4 100% identical to the homologous BTV-1 protein (BTV-8/1NS4), maintained the phenotype of wt BTV-8. Thus, it is possible that the role played by NS4 in counteracting the IFN response of the host could vary between different virus strains. It is important to stress that the strain of BTV-8 that we used in this study has been passaged only a few times in culture (once in KC cells and three times in BHK 21 cells) after isolation from blood of an infected animal. On the other hand, BTV-1 was derived from the ''reference'' South African strain passaged twice in embryonated eggs and 9 times in BHK 21 . BTV-1 appears to grow slightly faster than BTV-8 in culture, especially at the early time points post infection. Thus, faster replication may help BTV1-DNS4 to escape the IFN response of the cell more efficiently, as already suggested for some strains of influenza, and this may render NS4 less critical in these in vitro assays [80] .