The purpose of this study was to understand the stability and possibility of back mutation of Japanese encephalitis （JE） attenuated vaccine virus strain SA14-14-2 HK8 on molecular level. The E genes of the SA14-14-2 HK8 vaccine virus and its PHK cells passaged virus (SA14-14-2 HK17), its mouse brain passaged virus (SA14-14-2 SM1) were sequenced and compared with the E gene of parental SA14 virus. The total RNA was extracted from infected Vero cells and amplified by RT-PCR. The RT-PCR products were purified and cloned into T-vector. Positive clones were screened, identified and sequenced. There were twelve nucleotides and eight amino acids substitutions between SA14 parent virus and SA14-14-2 PHK8 vaccine virus. The SA14-14-2 PHK17 virus showed two additional mutations （E-331 and E-398） which were not back mutations. Although five additional mutations were found in SA14-14-2 SM1 virus, only one （E-107） was back mutation. Genetic characteristics of the attenuated vaccine virus SA14-14-2 were stable when it was passaged 17 times on PHK cells or one time in mouse brains.
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