The 181NVT glycosite, which is located at the apical b-folding of HA, is conserved in all human IVs (e.g., A/Anhui/1/2007 (H5N1)). Previous statistics have indicated that this glycosite has a lower level of conservation in avian IVs (e.g., A/chicken/ Vietnam/NCVD-093/2008 (H5N1)). The results of the molecular dynamics simulation indicated that the a2-3-sialoglycans adopted a straight-like and outward topology structure while the a2-6sialoglycans were fishhook-like and inward; therefore, we inferred that the that the deficiency of the glycosite would benefit the binding of SAa2-3Gal sialoglycans [68] . Actually, all the viruses that had deficiencies of the 181N glycosite were isolated from the avian host, which were concentrated in clades 7.1, 7.2 and 2.2.1.1 ( Figure 6A, File S3) .