The chemokine SDF-1alpha and its GPCR receptor CXCR4 have a one-to-one specificity that is unique among chemokines and their receptors. SDF-1alpha binds to CXCR4 expressed on hematopoietic and lymphopoietic cells, and directs their trafficking to and retention in hemato-and lymphatopoietic organs and sites of inflammation. CXCR4 is expressed on several tumor cell lineages, and might be responsible for metastasis to sites of SDF-1alpha expression, such as bone and lymph nodes. In addition, CXCR4 is a coreceptor for the HIV envelope glycoprotein gp120. Small molecule antagonists of CXCR4 have been developed and shown to inhibit infectivity of T-tropic HIV strains and to impair growth of brain tumors.