Modified HIV Env, Gag, Pol, or Nef DNA with improved ability to elicit antibody and CTL responses to HIV antigens have been identified as prototype immunogens for the treatment and prevention of HIV infections. Modifications to Env glycoprotein include deletions in the cleavage site, fusogenic domain, and spacing of heptad repeats 1 and 2 and different COOH-terminal deletions. Other modifications entail expression of Gag-Pol and Gag-Pol-Nef as fusion proteins.