RVFV has a tripartite negative-sense, single-stranded RNA genome with large (L), medium (M) and small (S) segments. The L segment encodes the viral RNA-dependent RNA polymerase, the M segment the two virion glycoproteins (G N and G C ) and the NSm nonstructural proteins, and the S segment the N nucleoprotein and the NSs nonstructural protein. Reverse genetic systems have been successfully developed for the recovery of recombinant RVFV (reviewed in [23] ). These rescue systems rely on transfection with plasmids expressing the three viral RNAs, and the N nucleoprotein and L RNA-dependent RNA polymerase, which are required for the packaging and replication of the viral RNAs. The RVFV RNA genome segments are expressed under the control of either the cellular DNA-dependent RNA polymerase I promoter [24, 25, 26] or the bacteriophage T7 promoter in cells that constitutively express T7 RNA polymerase [24] . Such rescue systems have been used to produce recombinant RVFV, including various mutants that lack the NSs, NSm genes, or carry specific mutations [26, 27, 28, 29, 30] . Viral strains that express reporter genes were also generated [24, 31, 32] . Importantly, in these recombinant viruses, the reporter gene activity directly reflects the extent of both viral transcription and replication.