Standard molecular biological techniques were used to construct plasmids. Total RNA of VeroE6 cells infected with RVFV ZH501, which was obtained from Stuart Nichol at the CDC, was used for first-strand cDNA synthesis. ZH501 virus stock at CDC [16] and our ZH501 stock underwent the same passage history. PCR fragments of full-length, antiviral-sense ZH501 S and L-segments were cloned into a pProT7 plasmid [45] , designated as pProT7-wS(+) and pProT7-wL(+), respectively. The ZH501 Msegment carrying G and A at nucleotide 847 of anti-viral-sense was cloned into a pProT7 plasmid, designated as pProT7-wM(+)-M847-G and pProT7-wM(+)-M847-A, respectively. The plasmids expressing ZH501 N or L proteins under the control of T7 RNA polymerase were reported previously [46] . The ORF of ZH501 glycoproteins, which carry G at nucleotide 847 of anti-viral-sense M were cloned into the pCAGGS plasmid under chicken b-actin promoter, designated as pCAGGS-G. All of the constructs were confirmed to have the expected sequences.