Structural analysis of amino acid changes in hemagglutinin in lineages B and C of influenza A(H7N9) viruses, China. Crystal structure of the homotrimeric H7 hemagglutinin bound to a human receptor analog (Protein Data Bank no. 4BSE) (27) (A) and rotated 90° counterclockwise (B) are shown. Two of the 3 protomers are displayed with high transparency to aid visualization. The carbon Cα positions of salient features are shown as spheres. Blue indicates receptor-binding residues, red indicates mutations in lineage B, green indicates mutations in lineage C, and orange indicates mutations in lineages B and C. Human receptor analog α2,6-SLN is shown as sticks colored according to constituent elements: carbon in orange, oxygen in red, and nitrogen in blue. Dark gray indicates the putative fusion peptide (32). Residues are numbered according to the H3 numbering system (Technical Appendix Table 2). A135 and L226 participate in receptor binding and thus are likely to modulate receptor specificity.