CXCL12 (stromal cell-derived factor-1, SDF-1) (Shirozu M et al, 1995) is produced in two forms, CXCL12alpha and CXCL12beta, by alternate splicing of the same gene (De La Luz Sierra M et al, 2004). It is a chemoattractant for T-lymphocytes and monocytes, but not neutrophils. In adult humans it plays an important role in angiogenesis by recruiting endothelial progenitor cells from the bone morrow through a CXCR4-dependent mechanism. CXCR4 (fusin) (Herzog H et al, 1993) is the receptor for CXCL12 and, like CCR5, is utilized by HIV-1 to gain entry into target cells. This receptor has a wide cellular distribution, with expression on most immature and mature hematopoietic cell types and also on vascular endothelial cells and neuronal/nerve cells. CXCR7 (RDC-1) (Infantino S et al, 2006) is expressed in monocytes, basophils and B cells and was originally an orphan receptor.