Structure of the Influenza A hemagglutinin monomer showing a representative extended cleavage site. A) The crystal structure representation of an uncleaved hemagglutinin HA0 precursor from subtype H3 (PDBID: 1HA0 , cleavage region green) overlaid with a structure of a cleaved HA1/HA2 heterodimer from a highly pathogenic H7 virus (PDBID: 4DJ6 , cleavage region blue) and an energy-minimized average representation of the cleavage loop for the highly pathogenic H7 with the new insertion modelled in YASARA Structure (yellow). Cleavage site indicated with dashed box at the stem region. B) Detailed structure of hemagglutinin cleavage site. Uncleaved loop (PDBID: 1HA0, green) shown with cleaved ends (PDBID: 4DJ6, blue) Glu317 of HA1 and Gly1 of HA2 connected with pink dots representative of the cleaved loop fragment. Basic arginine residues at positions 323, 326, 328 and 329 in the loop model are indicated.