To explore a novel biological function of RVFV NSs protein, in addition to its host transcriptional shutoff activity, we investigated the effect of ActD, a host transcriptional inhibitor, on the replication of MP-12 lacking the NSs gene in IFN-incompetent VeroE6 cells. VeroE6 cells were mock-infected or infected with MP-12 or rMP12-rLuc, which expressed Renilla luciferase (rLuc) in place of the NSs ( Figure 1A ) at a multiplicity of infection (moi) of 3. After 1 h virus adsorption, cells were incubated in the absence or presence of 5 mg/ml of ActD. Supernatants were harvested at 16 hours post-infection (h.p.i.), and virus titers were measured by plaque assay. ActD treatment had little effect on MP-12 titers, yet it significantly reduced the titer of rMP12-rLuc ( Figure 1B) , which suggested that the RVFV NSs was important for an efficient virus replication in the presence of ActD.