Roughly a 1-to-1 mixture of the rZH501-M847-G-type virus and rZH501-M847-A-type virus existed in the ZH501 virus stock. Because the original ZH501 sample from a patient in the 1977 RVFV outbreak was not available, it is unclear whether high titers of both rZH501-M847-G-type virus and rZH501-M847-A-type virus replicated in the patient. We found that the rZH501-M847-A-type virus became a major viral population in mice inoculated with rZH501-M847-G, while viruses accumulated in mice inoculated with rZH501-M847-A were the rZH501-M847-A-type virus. Additionally, 38 of 39 isolates from wt RVFV thus far reported were found to have A at M847 and carry Glu277 [19] , which is also encoded by rZH501-M847-A. These data suggest that the rZH501-M847-A-type virus probably represented the major virus population in the patient. However, it is possible that rZH501-M847-G-type virus existed as an RVFV quasispecies in the patient and affected the severity of the disease; quasispecies have cooperative interactions to control viral pathogenesis [20] . Consistent with this notion, we found a lesser virulence in a virus sample consisting of a major population of rZH501-M847-A-type virus and a minor population of rZH501-M847-G (Fig. 8A ) and obtained from a rZH501-M847-G-infected mouse brain at 8 days p.i., than did rZH501-M847-A or ZH501 (Figs. 3 and 9) . Although the accumulation of rZH501-M847A-type virus did not occur even after 5 passages of rZH501-M847-G in VeroE6 cells and MRC-5 cells, we observed that rZH501-M847-G replicated to higher titers than rZH501-M847-A in cultured cells (Figs. 2 and S1). This trend in virus replication in vitro pointed to the possibility that the rZH501-M847-G-type virus that was present as a minor virus population in the patient could have amplified in cell culture to become a major viral subpopulation in the ZH501 virus stock that was generated in vitro.