Restriction enzymes and DNA polymerases were purchased from New England Biolabs (Beverly, MA) or Takara (Dalian, China). DNase I was obtained from Worthington (Lakewood, NJ). Fluorescence-labeled secondary antibodies were purchased from Invitrogen (Shanghai, China) or Santa Cruz Biotechnology (Santa P5 (blue arrows), in relation to the five antigenic regions R1-R5. The coordinate of the HA protein is indicated by the grey bar, with the five antigenic regions represented in the same color as in Fig. 5 . The peptides P1-P5 were expressed as C-terminal fusions to the thioredoxin (Trx) tag. Binding activities of the peptides against the mouse (B), goat (C) and human (D) antisera before and after immunization were characterized by the method of ELISA, with the Trx protein as the negative control. The x-axis shows the dilution ratios of corresponding antiserum samples. The y-axis shows the absorbance at 450 nm after development with the substrate 3,39,5,59-tetramethylbenzidine (TMB). doi:10.1371/journal.pone.0018016.g006