Based on the results obtained with the HA mutants, an in silico analysis on the HA crystal structure of A/SC/1918 and A/PR/8/ 34 was carried out for the amino acid residues that the alanine scanning study showed to influence the binding of PN-SIA28 to HA. The analysis confirmed that the residues identified lie on the stem region of HA, that they belong to the HA1 and HA2 subunits and that they are exposed on the surface of the HA molecule ( Figure 3) .