Newly discovered structural characteristic of the gp120 V3 loop have resulted in a “rule” or algorithm, that is used in a method for determining whether a subject is infected with HIV-1 virus that expresses selectivity for CXCR4 or CCR5 chemokine receptors. A positively charged surface patch defined by V3 loop residues 11 and 24 or 25 at the base of the β-strands in the V3 loop and the homologous β2-β3 chemokine hairpin is responsible for CXCR4 receptor selection. Thus a method for detecting the presence of HIV-1 virus that is selective for X4-co-receptors in a subject infected with HIV-1 or suspected of being infected, from the amino acid sequence of at least a part of the HIV-1 gp120 V3 region peptide that includes residues 11, 24 and 25, or from the nucleotide sequence of a nucleic acid encoding said V3 region peptide, is disclosed.