Calculation of average d N /d S rates of evolution showed that both HA and NA genes evolved through negative (purifying) selection (Table 1) , with d N /d S values of 0.2762 and 0.1939, respectively. Even though in general, both genes underwent negative selection, some positions can evolve through positive selection. For example, an early study showed that two sites involved in receptor binding specificity of HA (220 and 278) were under positive selection, and these sites were not found in swine or seasonal H1N1 viruses [15] . Thus, changes in receptor binding sites could lead to alterations in receptor binding specificities.