In order to maximize the immunogenicity of HA DNA vaccines as shown in previous studies [9, 11] , HA genes used in the current study were also codon optimized and chemically synthesized. In addition, the HA gene sequences used in the current study express intact HA amino acid sequences at the cleavage site between HA1 and HA2 (PQREXRRKKRQG) of HA proteins in highly pathogenic H5N1 viruses [12, 13, 14] . This cleavage was shown to be important for the pathogenesis of H5N1 viruses [12, 15] . For inactivated H5 serotype flu vaccines, these residues were removed to improve the safety profile of such vaccines for both manufacturing and mass immunization purposes [7] [16, 17, 18] .