Next, we tested whether the co-expression of NSD249-265 can inhibit MP-12 replication. A549 cells were infected with MP-12 using an moi of 0.01, and then were either mock-transfected or immediately transfected with in vitro synthesized RNA encoding CAT (RNA transfection control), NSD249-265 or NSs-E253-255A (a control with functional NSs). Culture supernatants were harvested at 72 hpi for viral titration using plaque assay. RVFV titer was significantly decreased by the CAT RNA transfection when compared to mock-transfection control. Transfection with RNA encoding NSD249-265 did not further decrease RVFV titer compared to CAT RNA transfected control, while RNA encoding NSs-E253-255A increased RVFV titer significantly. These results suggest that co-expression of NSD249-265 NSs does not significantly decrease viral replication, while that of NSs-E253-255A facilitates RVFV replication by inhibiting IFN-b and PKR in transfected cells. Overall, co-expression of truncated NSs inhibited neither NSs functions nor RVFV replication efficiently. Even though NSs encode self-association domain at the Cterminus domain, the expressed protein mislocalizes in cells, and does not maintain the stability of intact NSs, which minimizes the dominant-negative effect on MP-12 NSs. Discussion Dominant-negative suppression of viral replication has been characterized in a number of different viral proteins [68, 69, 70, 71, 72, 73] . For RVFV, L proteins form oligomer, and exhibit dominant-negative function [74] . In this study, we used NSs protein as a target protein of dominant-negative suppression, because a lack of NSs dramatically attenuates RVFV [75, 76] .