The existence of N-glycosylation is necessary for viral membrane glycoproteins. The biosynthesis and modification of nascent secretory or membrane proteins occurs in the endoplasmic reticulum and Golgi, N-linked glycans encode crucial information for the folding, maturation, transport or degradation of proteins [29] . To escape both the host's humoral and cellular immune systems, the potential glycosites in viral envelope proteins can provide the identical glycans as those of the host's cells to mask the antigenic sites [28, 30] . Additionally, glycosylation also impacts the sensitivity of HA to temperature, the protection of cleavage sites and the stalk domain, and even the receptor-binding preferences [31] [32] [33] . As the ideal model for the influence of N-glycosylation in pathogen-host interaction, the present studies show that the envelope glycoproteins of IVs appear to only have N-glycosylation, with no O-glycosylation and GPI-anchors [34] ; hence, the glycosites discussed in this paper only pertain to N-glycosylation site. Figure 1 . The N-J trees of two glycoproteins in IVs with the corresponding distribution chart of glycosites. The phylogenetic trees of HAs and NAs were constructed using three to ten representative amino acid sequences in each subtype (File S1). The distribution charts of glycosites, colored according to the statistics of conservation in each HA or NA subtype (File S2), are shown in various strips. The red, green and blue color represent the levels of conservation of ''.95%'', ''5%,95%'' and '',5%'', respectively. The conserved cysteines are shown in yellow strips. (A) The N-J tree of HA subtypes with the corresponding distribution chart of glycosites. (B) The N-J tree of NA subtypes with the corresponding distribution chart of glycosites. doi:10.1371/journal.pone.0049224.g001