Chemokines are a family of small, structurally related molecules that regulate cell trafficking. We isolated a chemokine, stromal cell-derived factor/pre-B cell growth-stimulating factor (SDF-1/PBSF), as a molecule that stimulates the growth of B lymphocyte precursors and then found its multiple physiological functions in development. SDF-1/PBSF is essential for embryonic viability, B lymphopoiesis, bone marrow myelopoiesis, and cardiogenesis. Moreover, a primary physiologic receptor for SDF-1/PBSF is a seven-transmembrane G-protein-coupled receptor, CXCR4, that also functions as a coreceptor for strains of HIV-1. In recent years, we have shown that SDF-1/ PBSF and CXCR4 chemokine ligand receptor system is required for vascularization of the gastrointestinal tract by analyzing mice deficient in these molecules, thus defining a new signaling system for organ vascularization during embryogenesis.