Not all the glycosites shared the same numbering in one subtype. For this reason, we chose a representative sequence for each subtype (Table S1 ). Most glycosites were influenced by the deletion or insertion of the mutants in the upstream sequence, which is also known as ''antigenic drift'' [54] . In the HAs of the H1N1 or H5N1 viruses, some nearby glycosites were determined by ''antigenic shift'' [55] . In the H1 subtypes, the 144NVT glycosite from the seasonal H1N1 flu, which were isolated from 1940s to 1980s (e.g., A/Memphis/1/1984 (H1N1)), was replaced by the 142NHT glycosite after the 1990s (e.g., A/California/04/ 2007 (H1N1)). Additionally, no such glycosylation appeared in those H1N1 viruses isolated from swine H1N1 flu (e.g., A/swine/ Guangdong/1604/2010 (H1N1)), Spanish flu (e.g., A/South Carolina/1/1918 (H1N1)) and the 2009 influenza pandemic (H1N1) (e.g., A/Mexico city/CIA1/2009 (H1N1)). It has been partially shown that vaccination of mice with the 1918 influenza strain protected against subsequent lethal infection by the 2009 virus; however, the 1918 strain did not protect against the seasonal H1N1 flu [56] . It is interesting to note that the migration of the 179NKS to 177NLS tracks with these effects. Almost all of the 179NKS glycosites identified were centralized in swine-origin influenza viruses (S-OIVs, e.g., A/Swine/Guandong/1604/2010 (H1N1)) and parts of the 2009 influeza pandemic H1N1 virus (e.g., A/Mexico City/014/2009 (H1N1)), while the 177NLS glycosite was mainly found in the swine or human H1N2 viruses (e.g., A/ New York/481/2003 (H1N2)) and most seasonal flu viruses (e.g., A/California/04/2007 (H1N1)). These date further confirm that the HA of the 2009 influenza pandemic (H1N1) originated from S-OIV, not previous seasonal virus [57] . Similar positional conversion of the glycosites in H5N1 caused by ''antigenic drift'' was the ''179NYT/181NNT''. The conserved 181NNT glycosites have been replaced by the 179NYT in parts of clade 7 viruses since 2005. It reminds us that although these glycosites were adjacent, their N-glycans would shield different antigenic sites, which would provide some suggestions for the development of influenza vaccine.