In the single viral cycle assay, monomeric and trimeric C34 and N36 peptides were tested at various concentrations (10−6–10−10 M) for their capacities to inhibit the infection of HeLa-CD4-CCR5/CXCR4-LTR-β-gal cells with the X4 tropic HIV-1-VN44 or the R5 tropic HIV-1-BaL isolates over a period of 24 h. This evidence concerns the gene CXCR4 and infection.