The simultaneous presence or absence of four glycosylation sites (at residues 50, 58, 63, and 68) did not affect the infection of recombinant viruses in MDCK cells, while truncation of the NA stalk reduced the infection ability of the viruses (Figure 3), indicating that the HA/NA function balance of viruses with long-stalked NA is better than that of short-stalked viruses (Wagner et al., 2002; Murakami et al., 2008). This evidence concerns the gene XK and infection.