CXCR4 (fusin) is a members in the seven-transmembrane domain G-protein-coupled chemokine receptor family. The full-length cDNA was isolated from a human spleen cDNA library1. Its ligand is chemokine stroma-derived factor (SDF). CXCR4 has been shown to be the co-receptor for HIV-1 binding to CD4 through the viral envelope glycoprotein gp1202. In other cases, CXCR4 can function as the only receptor for the interaction of HIV-2 with CD4 and host cells3. A mouse monoclonal antibody (12G5) to CXCR4 has been shown to inhibit HIV infectivity and HIV-induced syncytium.