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Structural variation in the hemagglutinin (HA) stem and neighboring regions. Structures of HA protein from subtypes that have caused human infection are aligned by the helix A (HA2 residues 38–55) in the stem region: H1 (PDB 3LZG) [53], H2 (PDB 3KU5) [93], H3 (PDB 4FNK) [16], H5 (PDB 4BGW) [94], H6 (PDB 4XKD) [95], H7 (PDB 4LN6) [96], H9 (PDB 1JSD) [97], H10 (PDB 4XQ5) [98]. Zoomed-in views for several structural features of interest are shown. PDB, Protein Data Bank.
