We next examined whether the viruses that accumulated in infected mice retained the input-virus genome sequence at M847. To this end, we took advantage of the presence of a naturally occurring Sac I site (GAGCTC), which includes the A residue (underlined) at M847 of rZH501-M847-A. Due to single nucleotide substitution, the corresponding region of rZH501-M847-G lacked this restriction enzyme site. Mice were infected i.p. with 10 5 PFU of rZH501-M847-G or rZH501-M847-A (n = 9 for each group). Organs were isolated at various times p.i., and RT-PCR products that included M847 were subjected to Sac I digestion. For each virus group, we collected 3 livers at 4 day p.i., 3 spleens, 3 kidneys and 3 brains at day 6 p.i., and 3 brains at day 7 p.i. RT-PCR products of the expected size were obtained from all samples of rZH501-M847-A-infected mice, and all were fully susceptible to Sac I digestion (Fig. 7) , demonstrating that replicating rZH501-M847-A retained A residue at M847 in the infected mice. We were unable to obtain RT-PCR products from one spleen sample at day 6 p.i., one kidney sample at day 6 p.i., one brain sample at day 6 p.i., and one brain sample at day 7 p.i. from rZH501-M847-G-infected mice (data not shown), probably due to low virus titers in these samples. Two liver samples at day 4 p.i., were resistant to Sac I digestion (Fig. 7) . Unexpectedly, the RT-PCR products of one liver sample were a mixture of Sac Isusceptible PCR product and Sac I-resistant products (Fig. 7) and all other RT-PCR products, including two spleen samples at day 6 p.i., two brain samples at day 6 p.i., and two brains at day 7 p.i. (data not shown) were susceptible to the Sac I digestion (Fig. 7) . These data strongly suggested an accumulation of rZH501-M847-A-type virus carrying A residue at M847, in rZH501-M847-G infected mice at 4 days p.i. in livers and 6 days p.i. in spleens and brains.