The D253N Mutation in the Polymerase Basic 2 Gene in Avian Influenza (H9N2) Virus Contributes to the Pathogenesis of the Virus in Mammalian Hosts

禽流感（H9N2）病毒聚合酶碱性蛋白2基因中的D253N突变是导致该病毒在哺乳动物宿主中致病的原因之一。