Schematic representation of the positions of amino acid substitutions involved in Stachyflin resistance. Three-dimensional image of the H1 HA molecule was created with data from X-ray crystallography of PR8 (PDB code: 1RU7) in the Protein Data Bank Japan and Discovery Studio Visualizer 1.6. Yellow spheres on the HA molecule indicate the positions of amino acid substitutions in Stachyflin-resistant virus clones of WSN selected in the presence of Stachyflin, and red sphere indicates that of PR8. Orange sphere indicates the position of amino acid substitution observed in both Stachyflin-resistant virus clones of WSN and that of PR8. The positions of amino acids correspond to the H3 HA numbering.