We first screened the yeast library displaying the H1N1 HA peptides to study the murine immune response to recombinant HA protein of the 2009 A(H1N1) Influenza virus. Freshly induced library cells were incubated with mixed sera from immunized mice and subsequently labeled with fluorescent secondary antibodies. To avoid potential bias introduced by growth advantage of the yeast cells, only one round of sorting was performed. Plasmids isolated from positive yeast cells were transformed into E. coli cells and plated. Single clones (100) were picked randomly and sequenced, of which 82 (82%) were shown to contain in-frame inserts. These results indicate that the sorting process is able to distinguish in-frame sequences from the random library that contained only 1/6 (16.7%) in-frame inserts efficiently. After verification of individual peptides by FCM detection, the ratio of recombinant antigen expressed on the yeast surface (RAYS) was determined by dividing the mean fluorescence intensity of peptide expressing yeast cells by the mean fluorescent intensity of the nonexpressing (pCTCON-2) yeast cells [26] . 56 positive clones with a RAYS ratio $2 were considered as antigenic and analyzed at both the sequence and structural level [26] . Alignment of positive peptides with the original HA sequence shows that 87% are in the HA2 region (95627 aa), with just 13% in the HA1 region (259626 aa) which cover nearly the entire HA1 protein (Fig. 2, panel A). The FCM histograms of several representative yeast clones are shown in Fig. 3 , panel A. Clones M-5 (corresponding to residues 55-313 of HA, or 70 aa shorter than HA1) and M-7 (residues 69-275, or 120 aa shorter than HA1) were strongly positive (RAYS ratio .5), and contained the entire receptor binding domain (RBD) and the vestigial esterase domain in the globular HA head. Clones M-33 (residues 387-489) and M-52 (residues 423-492) displayed much shorter peptides but also showed strong fluorescent FCM signals (Fig. 3 , panel A), albeit with relatively lower RAYS ratios of 3-4. Judged from the mean fluorescence intensities, the binding affinities of these displayed peptides were generally weaker than the whole HA1 (Fig. S2,  panel B) .