The 'H' in H1N1 (and all forms of influenza). This variant is based on the PDB structure 1RUZ from the 1918 pandemic, and is colored as follows to indicate sites of mutations between different variants.

Blue: The receptor binding site, amino acids 98, 134, 135, 138, 153, 155, 183, 190, 194, 224, 225, 227, and 228.

Red: Amino acids associated specifically with the 1918 pandemic (residues 222 and 187).

Yellow: Glycosylation site (Asn279) which is thought to be related to increased transmission and virulence in the 2009 pandemic.

Orange: Binding-site epitope residues Thr136, Ala137, Gln226 for human antibody 5J8 (PDB structure 4M5Y) that potently neutralizes a broad spectrum of H1N1 viruses, including the 1918 and 2009 pandemic viruses.

Green: Additional contact point for antibody 5J8.