Although P3 (residues 38-52, indicated by blue) and P31 (residues 318-332, indicated by red) are parts of HA1 in primary sequence, they are located in the middle of helix A and B in the trimeric structure and are partially surface exposed. P5 (residues 58-72, indicated by magenta) seems to be a dispatch that links the stem region and the globular region and is fully surface exposed (Fig. 1B) . P15 and P16 (residues 158-172 and 168-182, indicated by orange) are located in the receptor binding domain [11] .