Recombinant NP was expressed from pET16b in BL21-condon plus (DE3)-RIL cells (Stratagene, La Jolla, CA, USA). Soluble NP was purified by a His-Bind Kit (Novagen, San Diego, CA, USA) and purified proteins were analyzed and confirmed by SDS-PAGE. Western blotting analysis was performed using an anti-influenza A NP antibody (Chemicon, Billerica, MA, USA).