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3-D models of the receptor-binding site of the HA molecules of A/Victoria/210/2009 (vaccine original virus) and X-187 (vaccine virus). The 3-D model structures of the virus HAs were constructed by homology modeling as described in Materials and Methods. Between the A/Victoria/210/2009 and X-187 viruses, only one amino acid at residue 228 located in the receptor-binding site was different.
