The HA polypeptide is cleaved into two subunits linked by a disulfide bond, HA1 and HA2, to form the mature HA trimer. The majority of the HA1 subunit forms the viral membrane-distal globular head responsible for receptor binding, whereas the HA2 subunit together with the N-and C-termini of HA1 forms the membrane-proximal stem region that plays a major role in membrane fusion. Thus two plasmids, pCTCON-HA1, pCTCON-HA2, respectively, were constructed to display the HA1 subunit of 328 aa (residues 17-344 of the HA protein) and the HA2 ecto-domain of 192 aa (residues 345-536 of the HA protein) as positive controls for FCM (Fig. 1) . The HA1 and HA2 displayed on the yeast surface were recognized by the mouse, goat and human antisera, and confirmed by FCM (Fig. S2 , panels B, E, and, H; and C, F, and I, respectively). Screening of antigenic peptides recognized by mouse antisera immunized with HA protein