In accordance with previous reports [33], [47], at the early stage of viral infection, the titers of A+S− and A+S+ in MDCK cells were approximately 1.5 log10 TCID50/ml higher than those of A−S− and A−S+, which indicates that the replication ability of the viruses with a long-stalk NA in MDCK cells was better than that of the viruses with a short-stalk NA. This evidence concerns the gene XK and viral infectious disease.