CXCR4, a Chemokine Receptor involved in organ vascularization, neuronal cell migration, and patterning of the central nervous system during development. It binds stromal cell-derived factor 1 (SDF1, also called PBFS) and mediates migration of resting leukocytes and is unique in homing hematopoietic progenitors to bone marrow. These properties suggest that CXCR4 is involved in tumor cell migration and local tumor invasion. It is also an HIV-1 fusion co-factor that allows HIV-1 invasion in diverse human cell types. Although this receptor was initially called Neuropeptide Y3 Receptor, it does not respond to neuropeptide Y. CXCR4 has two isoforms that are produced by alternative splicing. CXCR4 has been reported to be expressed in cells of the immune and central nervous systems. It is highly expressed in brain and heart, and in white blood cells, vascular endothelial cells, and umbilical cord endothelial cells. In brain tumors, CXCR4 is expressed in regions of angiogenesis and degenerative, necrotic, and microcystic changes (Rempel et al., 2000). ESTs have been isolated from a wide variety of tissue libraries, particularly uterus, kidney, lung, tonsil, and brain.