The hemagglutinin (HA) protein attaches viral particles to host cells by binding to sialic acid-containing receptors and induces viral particle internalization through clathrin-dependent endocytosis. HA also promotes an alternative clathrin- and caveolin-independent pathway for some virions. Hemagglutinin/HA Protein, H5N1 (R339G, S341R, sf9, His) is the recombinant Virus-derived Hemagglutinin/HA protein, expressed by Sf9 insect cells , with C-His labeled tag.