SDF-1b (stromal cell-derived factor-b), also known as pre-B-cell growth-stimulating factor (PBSF), is a 74-amino acid CXC chemokine originally cloned from a bone marrow stromal cell line. Targeted deletion of SDF-1 gene resulted in defects of B-cell lymphopoiesis and bone marrow myelopoiesis in mice. SDF-1 has been shown to be chemotactic for lymphocytes. In addition, SDF-1 was recently reported to be a ligand for CXCR4 (LESTR/fusin), a co-receptor for HIV-1 entry into T cells. SDF-1 binding to CXCR4 inhibits HIV-1 entry.