Stromal cell-derived factors 1-alpha and 1-beta, also known as (CXCL12), are small cytokines that belong to the chemokine family, encoded by the CXCL12 gene. The chemokines are characterized by the presence of 4 conserved cysteines that form 2 disulfide bonds. CXCL12 is strongly chemotactic for lymphocytes. During embryogenesis it directs the migration of hematopoietic cells from foetal liver to bone marrow and the formation of large blood vessels. CXCL12 also has a role in tumor metastasis where cancer cells that express the receptor CXCR4 are attracted to metastasis target tissues that release the ligand, CXCL12. CXCL12 is expressed in the area of inflammatory bone destruction. It is chemotactic for mesenchymal stem cells and mediates their suppressive effect on osteoclastogenesis.