For the purpose of investigating the co-evolution relationship between the glycosite patterns of HA and NA, we reconstructed a phylogeny tree of HAs and determined ten clades. When discrete monophyletic groups with a common node meet a bootstrap value of $60 at the clade-defining node and average percentage pairwise nucleotide distances between and within clades of .1.5% and ,1.5%, respectively, they could split into the second-, third-, or even fourth-order clades [50] . The statistics of the glycosites of each HA, as well as the corresponding NA, were recorded according to their clades (Files S3, S4 and Figure S1 ).