Interestingly, the deletion of the stalk domain in H5N1 NA is variable. As the first human death reported in 1997, most H5N1 viruses that belonged to clade 0 and isolated in Hong Kong were characterized by ''N-Q-S-I-54I-18-73N-F-Y'', which remained a glycosite: 50NQS. Although previous studies conjectured that the H6N1 virus (e.g., A/Teal/Hong Kong/W312/97 (H6N1)) was the donor of the NA gene in 1997 HPAI virus [69, 70] ; however, similar motif could be found even in 1983. Since 2000, the most common pattern of ''A-E-48P-20-69I-S-N'' with four glycosites missing has dominated in the H5N1 NAs ( Figure 6B, File S4 ).