Gene schematic diagrams and 3D-structure of H9N2 neuraminidase used in this study. (A) Gene schematic diagrams of SD01, SS94 and recombinant virus. The blue and red bars indicate the genes originating from SD01 and SS94, respectively. The positions of four amino acid regions in the NA gene are shown at the top of the diagram and differences in the mutants are shown as amino acid abbreviation. Amino acids in NA of SD01 and SS94 are marked up by yellow and black letters respectively. + indicates that the virus could be transmitted among chickens via aerosols, − indicates could not. (B) The 3D-structure of H9N2 neuraminidase generated using PyMOL software shows the locations of mutations (PDB access number: 1ivd). The left is the structure before the mutation, and the right is the structure after the mutation. * indicates the positions that will be studied. Amino-acids 368–370 are close to the HB site, while 313–381 are on the opposite side of the globular head.