Vero E6 cells, wild-type mouse embryonic fibroblast (MEF) cells and Pkr 0/0 MEF cells [37] were maintained in Dulbecco's modified minimum essential medium (DMEM) (Invitrogen) containing 10% fetal bovine serum. BHK/T7-9 cells [63] , which stably express T7 RNA polymerase, were grown in MEM-alpha (Invitrogen) containing 10% fetal bovine serum (FBS). Penicillin (100 U/ml) and streptomycin (100 mg/ml) (Invitrogen) were added to the media. BHK/T7-9 cells were selected in medium containing 600 mg/ml hygromycin (Cellgro). An RVFV vaccine candidate MP-12 [13] and recombinant MP-12 [19] were used for the experiments, and infectivity was assessed by a plaque assay in Vero E6 cells.