CXCR4 (Chemokine,CXC Motif, Receptor 4), also known as FUSIN or NPY3R, is a protein that in humans is encoded by the CXCR4 gene. By PCR analysis of human/hamster hybrid cell DNA, Herzog et al. (1993) showed that the NPY3R gene is located on human chromosome 2. Feng et al. (1996) showed that transient expression of the CXCR4 gene allowed nonhuman cells coexpressing recombinant CD4 to undergo Env-CD4-mediated cell fusion and productive HIV-1 infection. Peled et al. (1999) demonstrated that SDF1 and its receptor CXCR4 are critical for murine bone marrow engraftment by human SCID repopulating stem cells. Treatment of human cells with anti-CXCR antibodies prevented engraftment.