The generation of a recombinant RVFV expressing a green fluorescent protein (GFP), rZHDNSs-GFP, has previously been described [24] . Our previous attempts to generate a recombinant RVFV expressing a humanized firefly luciferase (hFLuc) have been confounded by genetic instability and the rapid emergence of mutants with deletions [24] . Therefore, we generated rZHDNSs-hRLuc RVFV that carries a humanized Renilla luciferase (hRLuc) gene using Pol I based plasmids as previously described [24] . The rescued rZHDNSs-hRLuc was amplified in Vero E6 cells and stocks produced. The titer reached 8610 7 PFU/ml. The plaques formed by rZHDNSs-hRLuc were fuzzy with a faint staining inside, as those obtained with the rZHDNSs virus that carries a deletion of the NSs gene [24] . To test the luciferase expression within the infected cells, Vero E6 cells were infected with rZHDNSs-hRLuc at a MOI of either 0.3 or 3, and lysed every 2 hours for 12 h. Luciferase activity was measured using coelenterazine, a specific substrate of Renilla luciferase, and found to be expressed at significant levels from 2 h post-infection onwards, while uninfected Vero E6 cells showed no luciferase activity. The luciferase activity increased with time and was dependent on the MOI (data not shown). This is consistent with previous reports [31, 32] . To check the stability of the recombinant virus in vitro, rZHDNSs-hRLuc was passaged on Vero E6 cells and, at each passage, we measured the viral titer in the supernatant at 72 h post-infection and the luciferase activity within the infected cells at 8 h post-infection with a MOI of 3. Both the viral titer and the luciferase activity remained stable over at least 8 passages, varying from 10 7 to 10 8 PFU/ml and from 10 7 to 10 8 raw light units (RLU)/s per 3610 5 cells, respectively. To test the virulence of the recombinant virus, wild-type 129S2/SvPas mice (N = 5) and 129S2/SvPas mice deficient for IFN-a/b receptor subunit 1 (Ifnar1 2/2 ) (N = 10) were infected i.p. with 10 4 PFU of rZHDNSs-hRLuc. All wild-type mice survived the infection for 13 days with no signs of disease, as seen following infection with rZHDNSs in which the NSs gene is totally deleted [24] or Clone 13, a natural isolate that lacks 69% of the NSs open reading frame [33] . In contrast, all rZHDNSs-hRLuc infected Ifnar1 2/2 mice died within 45 h (Figure 1 ) from severe hepatitis with no signs of neurological disorder. Infections of Ifnar1 2/2 mice with rZHDNSs or with Clone 13 gave similar results ( [24] and data not shown).