Provided is a polypeptide having no more than 100 amino acids, which polypeptide comprises one or more sequences having at least 60% homology with any of SEQ ID 1-4, or comprises two or more epitopes having 7 amino acids or more, each epitope having at least 60% homology with a sub-sequence of any of SEQ ID 1-4 that has the same length as the epitope: SEQ ID 1 GDTWAGVEAIIRILQQLLFIHFRIGCQHSR SEQ ID 2 KVGSLQYLALTALITPKKIKPPLPSVKKLTEDRWNKPQKT SEQ ID 3 EPVPLQLPPLERLTLDCSEDCGTSGTQ SEQ ID 4 YKGALDLSHFLKEKGGLEGLIYSQKRQDILDLWVYHTQGYFPD wherein, the polypeptide is immunogenic in a vertebrate expressing a major histocompatibility complex (MHC) allele, and wherein the polypeptide is not a complete HIV virus protein.