We subsequently investigated whether ORFX was a structural or non-structural protein. We infected BFAE cells with BTV-1 and analysed supernatants (containing viral particles) and total cellular protein extracts. Unusually for mammalian cell lines, BFAE cells show very little BTV induced cytopathic effect (CPE), thus facilitating the efficient discrimination between all BTV proteins present in the cellular fraction and the structural proteins present in purified and concentrated viral particles released from infected cells. By western blotting, we detected NS1 and ORFX in the cellular fraction, while VP7 was abundantly present in the viral fraction (concentrated by ultracentrifugation) and barely visible in the cellular fraction ( Figure 1C) . We obtained the same results by infecting C6/36 mosquito cells (data not shown). We detected VP6 in both the cellular and the viral fraction ( Figure 1C) . Interestingly, unlike VP7, VP6 appeared to be relatively more abundant in cell lysates compared to the viral pellets, suggesting that there is an intracellular pool of this protein that is not incorporated in the BTV virions.