Chemokine (C-X-C motif) ligand 12 (CXCL12) also known as stromal cell-derived factor 1 (SDF-1) is a chemokine protein that activates leukocytes and is often induced by proinflammatory stimuli such as LPS, TNF or IL-1. 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. In adulthood, CXCL12 plays an important role in angiogenesis by recruiting endothelial progenitor cells (EPCs) from the bone marrow through a CXCR4 dependent mechanism. CXCL12 also has a role in tumor metastasis where cancer cells that express the receptor CXCR4 are attracted to metastasis target tissues. In breast cancer, however, increased expression of CXCL12 determines a reduced risk of distant metastasis.