The significance of the conserved glycosites in the stalk domain was providing the N-glycans to avoid the cleavage by host enzyme (e.g., Trypsin) [60, 61] . The variance of the glycosites was closely related to the deletion of the stalk domain. Although the threedimensional structure of stalk domain has not been determined yet, it is speculated that the presence of an a-helix motif in the uncrystallized structure has also been provided by cryoelectron microscopy [62] . Wagner et al. believed that a longer stem domain would enhance the replication capacity of the virus, while the deletion of the stem domain would decrease the enzymatic activity of NA [63] . Various subtypes had stalk domain deletions of 3 to 24 residues, except for N4, N8, N9 and N10. The numbers of deletions were also distinctive across different combinations of IVs and even within one subtype, such as the N2 subtype. Generally, no deletions were found in the NA of the H3N2 virus; deletion of 3 residues and one corresponding glycosite with the pattern ''E-R-61N-3-64T-V-H'' (meaning 3 residues missing between 61N and 64T, e.g., A/chicken/Zhejiang/611/ 2011 (H9N2)), appeared in most of the NA subtypes of the H9N2 virus. A similar deletion of 20 residues and two glycosites (I-E-60R-20-80N-I-I) appeared in most NA subtypes of the H6N2 virus (e.g., A/duck/Fujian/3193/2005 (H6N2)). The deletion of 16 residues and two glycosites as ''C-E-55P-16-72T-T-E'' were a distinctive part of the H7N2 virus (e.g., A/unknown/New York/19501-5/2006 (H7N2)). Parts of the H5N2 virus were characterized by the deletion of 20 residues and two glycosites (''R-N-62I-20-83G-Y-R'', e.g., A/chicken/Ibaraki/3/2005 (H5N2)). Moreover, in some avian H2N2 viruses, the deletion of 22 residues resulted in the loss of two glycosites; however, a new glycosite appeared in the newly created sequences (''P-A-47N-22-70N-T-V'', e.g., A/chicken/New York/Sg-00300/1997 (H2N2)). In all, the diversity of deletions in the stalk domain indicates that the glycosylation pattern of HA and NA has a complex relationship.