IAV escapes the human immune system by continuous antigenic drifts and occasional antigenic shifts [33] . Attempts to develop universal vaccines and reliable diagnostic tools based on conserved epitopes of IAV are big challenges. Several epitopes that can elicit broad spectrum neutralizing antibodies have been identified recently. For example, Sui et al. identified a universal neutralizing epitope for group 1 HA [34] . Yoshida et al. reported a universal epitope in antigenic site B shared by H1, H2, H3, H5, H9, and H13 subtypes [35] . All these epitopes are conformationdependent. In this study, we identified two epitopes (P5 and P31) which have not been identified previously (Fig. S1 ). The P5 (aa58-72) seems to be a dispatch that links the stem region and the globular region and is fully exposed on the surface, while P31 (aa 318-332d) is located in the middle of helix A and B on HA2. In contrast to previous studies, we found P5 to be a linear B cell epitope. Our data demonstrate that this epitope is highly conserved among H1 viruses (9860/10767, 91.6%). Because viral mutants that are resistant to conformational epitopes are more easily generated, the conserved linear epitope is more suitable for differentiating subtypes than conformational epitopes [33] . Hence, the epitope in P5 provides a new target for reliable diagnostics of H1-subtype IAVs.