A recombinant MP-12 carrying the PKRDE7 ORF, rLuc-Flag or the NSs-Flag in the place of the NSs ORF was recovered as described previously [19] . Briefly, subconfluent monolayers of BHK/T7-9 cells were co-transfected with an S-genome RNA expression plasmid, such as pProT7-S(+)-PKRDE7, pProT7-S(+)-rLuc-Flag or pProT7-S(+)-NSs-Flag, and a mixture of pPro-T7-M(+), pPro-T7-L(+), pT7-IRES-vN, pCAGGS-vG, and pT7-IRES-vL using TransIT-LT1 (Mirus Bio Corporation). The culture medium was replaced with fresh medium 24 h later. At 5 days post-transfection, the culture supernatants were collected, clarified and then inoculated into VeroE6 cells. The supernatant of infected VeroE6 cells at 3 days post-infection was used for the experiment.