CXCR-4 (fusin or LESTR) is a G-protein linked seven transmembrane spanning chemokine receptor that binds stromal cellderived factor-1 (SDF-1) (1, 2). Additionally, CXCR-4 has been found to act as a co-factor for the T-cell tropic HIV-1 and -2 virus entry into cells (3, 4). A monoclonal antibody (clone 12G5) to the CXCR-4 structure was developed by Enders et al. (4) and shown by flow cytometry applications to react with CXCR-4 expressed on a variety of human cell lines, including Sup-T1, Hut-78, Molt4, CEMss, Daudi and Hela, as well as with peripheral blood lymphocytes (4). Clone 12G5 can neutralize infection and inhibit syncytium formation induced by the HIV virus (4). The epitope for the 12G5 monoclonal antibody maps to a region of the CXCR-4 receptor that includes extracellular loops 1 and 2, with the first extracellular loop playing a more critical role in the epitope structure (5). R&D Systems has developed a series of monoclonal antibodies that specifically react with the human CXCR-4 structure on transfected cells as well as the CXCR-4 antigen on both stimulated and unstimulated peripheral blood lymphocytes. Clone 44717.111 appears to react preferentially with the second external loop region of the CXCR-4 receptor. CXCR4 Customer Publications.