The H9N2 subtype of avian influenza viruses have widely circulated in mainland China since 1994.Previous studies revealed that the H9N2 viruses in mainland China formed multiple genotypes.The surface protein hemagglutinin(HA)gene could be divided into three sub-lineages represented by A/ehicken/Beijing/1/94(BJ/94),A/quail/Hong Kong/G1/97(G1)and A/chicken/ Heilongjiang/35/01,and the neuraminidase(NA)gene could be divided into three sub-lineages represented by A/chicken/ Beijing/1/94(BJ/94),A/quail/Hong Kong/...

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