The Human Immunodeficiency Virus Type 1 (HIV-1) isolates have been found to be highly divergent both in nucleotide and amino acid sequences. Phylogenetic analysis of the worldwide distributed isolates showed at least nine HIV-1 subtypes (A-I) included in the majoritary M group, in addition to the highly divergent group O (G Myers et al. 1994 Human retroviruses and AIDS: A Compilation of Nucleic and Amino Acid Sequences, Los Alamos, Natl Lab Los Alamos, New Mexico, W Janssens et al. 1994 AIDS Res Hum Retrov 10: 877-879, LG Kostrikis et al. 1995 J Virol 69: 6122-6130). This striking genetic variability of HIV-1 isolates, specially at the immunologically important regions of the envelope protein gp120, can be limiting for the development of a broadly protective vaccine. Indeed, one of the important antigenic epitopes recognized by neutralizing antibodies, is the type-specific principal neutralizing determinant (PND) which maps in the top of the V3 loop (K Javaherian et al. 1989 Proc Natl Acad Sci USA 86: 6768-6772, K Javaherian et al. 1990 Science 250: 1590-1593, GD La Rosa et al. 1990 Science 249: 933-935).