Since the NA activity of influenza viruses can affect the HA–NA balance, which in turn affects the initiation of viral infection, viral fitness, and cross-species transmission [24], we measured the HA titers of these RG pH1N1 viruses using turkey RBCs in the presence or absence of 10 μM oseltamivir carboxylate at 4 °C or 37 °C. The gene discussed is XK; the disease is viral infectious disease.