Sequence analysis of the RT-PCR products of intracellular ZH501-specific RNAs showed sequence homogeneity in all three RNA segments, except for nucleotide 847 of the anti-viral sense M segment RNA (M847), which consisted of a mixture of A and G residues (Fig. 1) . Of 35 independent, cloned RT-PCR products of a region including M847, 18 clones and 17 clones had A and G, respectively, at M847, which suggested to us that the ZH501 virus stock consisted of roughly a 1-to-1 mixture of two major viral subpopulations, one carrying A and the other carrying G at M847. The virus carrying A residue at M847 encodes glutamic acid at amino acid 277 (Glu277) of the Gn envelope protein, while that carrying the G residue encodes glycine (Gly277). Sequence analysis of the cloned PCR products also showed other sequence heterogeneities, including silent mutations and those that altered amino acid sequences, within the M segment (data not shown), revealing a quasispecies nature of the ZH501 virus stock. None of these other nucleotide heterogeneities were shared among clones (data not shown).