BTV-8DNS4 consistently reached lower titres (approximately 10 to 25 fold) than wt BTV-8 in cells treated with 1000 AVU/ml of either IFNT or UIFN (Figure 7) . Similar to what was observed in the IFN protection assays, there was no discernable difference in the replication growth of BTV-1 and BTV-1DNS4 after treatment with either IFNT or UIFN. Similar patterns with both BTV-1 and BTV-8 wt and the DNS4 mutant viruses where observed when the input viruses were used at a MOI of 0.1 and 0.001 in CPT-Tert (data not shown), or in BFAE cells treated with UIFN and infected at a MOI of 0.1 (data not shown).