However, functional antibody analyses with the rabbit anti-HA immune sera showed a very different picture when these sera were further analyzed by either the hemagglutination inhibition (HI) or microneutralization (MN) assays. All three forms of H5-HK HA DNA vaccines induced protective antibody responses against the autologous wild type virus A/HongKong/483/97, but levels of protective antibodies were different among sera induced by different designs of H5-HK HA DNA vaccines. The DNA vaccine with the full length HA insert under the tPA leader sequence (H5-HK.tPA) elicited consistently higher HI and MN antibody titers when compared to the other two forms of HK-HA inserts, the full length HA with a natural leader sequence (H5-HK.wt), and the transmembrane region (TM) truncated HA (H5-HK.dTM). The difference was statistically significant (p,0.05) between H5-HK.tPA and H5-HK.dTM sera based on the HI assay (Fig. 2D ) and between H5-HK.tPA and H5.HK.wt sera based on the MN assay (Fig. 2E) . Expression and immunogenicity of different forms of DNA vaccines coding for the HA antigen of an H5N1 isolate A/VietNam/1203/04