Finally, we examined mouse virulence of rZH501-M847-Atype virus that had accumulated in rZH501-M847-G-infected mice by i.p. inoculation of 10 1 , 10 2 , 10 3 , 10 4 or 10 5 PFU of the brain homogenates of 8b mouse (see Fig. 8A ) into mice (n = 5 for each dilution); RT-PCR analysis of mouse 8b brain extracts revealed that rZH501-M847-A-type virus was the major virus population, and sequence analysis of all 20 cloned RT-PCR products of this brain extract showed that M847 was A. Inoculation of 10 4 or 10 5 PFU of the recovered virus resulted in the death of all mice within 7 days p.i., while 2 mice, 3 mice and 4 mice survived after inoculation of 10 3 PFU, 10 2 PFU and 10 1 PFU of the virus, respectively (Fig. 9 ). The recovered viruses had a higher mouse virulence than did rZH501-M847-G, while they were not as virulent as rZH501-M847-A or ZH501 (Fig. 3) .