This invention provides a DNA which upon transcription produces an RNA encoding a modified HIV-1 gp140 polypeptide, which polypeptide upon cleavage produces a modified gp120 and a modified ectodomain of gp41 which together form a complex exhibiting enhanced binding to HIV-1 neutralizing antibodies and reduced binding to HIV-1 non-neutralizing antibodies, wherein the modifications comprise an A492C mutation in gp120 and a T596C mutation in gp41, said mutations being numbered by reference to the HIV-1 isolate JR-FL, and resulting in a disulfide bond between gp120 and ectodomain gp41 which stabilizes the otherwise non-covalent gp120-gp41 interaction.