The levels of pairwise nucleotide conservation at synonymous sites within aligned sequences of the VP6 ORF, were used to assess the functional importance of the NS4-ORF. Complete or near-complete VP6-encoding sequences from BTV showed strikingly enhanced conservation in the region corresponding to the NS4 ORF (figure 1), supporting and extending previous computational analyses [17] . Enhanced conservation was also apparent at the 59 end of the VP6 coding sequence, indicating that this region (like the terminal noncoding regions of orbiviruses) is likely to contain functionally important elements.