Although the 15D peptide had reduced ability to block binding of the 12G5 antibody, probably because it has a negative Asp residue instead of Arg and it is known that the chemokine binding site of CXCR4 is negatively charged, the infectivity data indicate that the low-affinity interaction of 15D was still sufficient to interfere with viral infection. Here, CXCR4 is linked to viral infectious disease.