HA/hemagglutinin proteins bind to sialic acid receptors and initiate viral attachment and internalization through clathrin-dependent or -independent pathways. This class I viral fusion protein determines viral host range and virulence. HA/Hemagglutinin Protein, H1N1 (sf9, His) is the recombinant Virus-derived HA/Hemagglutinin protein, expressed by Sf9 insect cells , with C-His labeled tag.