Representative antigenic clones were subjected to FCM. As shown in Fig. 3 , panel B, clones G-15 (residues 38-136 of HA) and G-29 (residues 70-162) in the HA1 region, G-46 (residues 308-339) near the HA1-HA2 cross region, and G-55 (residues 413-479) in the HA2 region, all shorter than 100 aa, demonstrated strong antigenicity (RAYS ratio .5). Judged from the mean fluorescence intensities, the binding affinities of these displayed peptides were generally stronger than the whole HA1 (Fig. S2 , panel E).