M2e protein induces neutralization of influenza virus in mice. 1×M2e, 4×M2e, a-H9N2, a-H9N2/1×M2e, or a-H9N2/1×M2e was used to immunize mice. Three weeks after boost vaccination, sera were collected. The samples were serially diluted two-fold. Serum neutralization activity was tested against 102 TCID50/ml of respective viruses. (A) A/chicken/Korea/ma163/04 (ma163/H9N2), (B) A/aquatic bird/Korea/maW81/05 (maW81/ H5N2), or (C) A/Philippines/2/82 (Phil82/H3N2) virus for 30 min, followed by incubation with MDCK cells for 48 h. Data are representative of three independent experiments with three replicate wells per group. The lower limit of detection (0.5 10 log2TCID50) is indicated by a dotted line.