Based on the results obtained from the alanine scanning study, an in-silico analysis on the HA crystal structure of A/PR/8/34 (PDB ID code 1RU7) was carried out. 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 4 ).