To define the epitope contained in P5 precisely, a peptideinhibition ELISA was performed. This experiment is reliable and is a standard methodology to determine the fine specificity of antigen-antibody reactions [31, 32] . A panel of short peptides derived from P5 (N6-N14, with C-terminus truncation; and C6-C14, with N-terminus truncation) were used to block the binding of anti-P5 antibody to coated P5. As shown in Fig. 5 , antibody induced by the P5-KLH conjugate was inhibited by peptide N10-N14 and the parental peptide P5 to similar extents, whereas peptides N6-N9 only showed inefficient inhibition even at high molar concentrations. A similar pattern of inhibition was observed with the C-terminal conservative derivatives. Peptides C12, C13, C14, and the parent peptide P5 demonstrated comparable and efficient inhibition, whereas only slight inhibition was observed in peptides C6-C11 (Fig. 5B) . Since the amino acid sequence LRGVAPL overlapped both peptides N10 and C12 (Fig. 5) , we speculate that this sequence met the minimum requirements of binding to the anti-P5 antibody. However, the synthetic peptide LRGVAPL did not block the binding between P5 peptide and its antibody, nor did it directly bind to the P5 antibody (Fig. 6 ). As P5 (1918, 1934, 1977, 2007, 2009 ) and a swine H1N1 strain. 293T cells transfected with an empty vector was used as a control (Ctrl). b-actin was used as a loading control. For the backgrounds of various subtype IAV strains, see 