A modified polypeptide corresponding to an envelope glycoprotein of a primate lentivirus is described. The polypeptide has been modified from the wild-type structure so that it has cysteine amino acid residues introduced to create disulfide bonds, a cavity is filled with hydrophobic amino acids, a Proresidue is introduced at a defined turn structure of the protein, or the hydrophobicity is increased across the interface between different domains, while retaining the overall 3-dimensional structure of a discontinuous conserved epitope of the wild-type protein. Preferably, the polypeptide has more than one of those characteristics. Preferably, the primate lentivirus is HIV, and the protein is HIV-1 gp120.