CXCR4 and breast cancer: With respect to the last-mentioned, the chemokine CXCL12/SDF-1 (stromal cell-derived factor-1) and its receptor CXCR4 have attracted great interest as they direct CXCR4-expressing breast cancer cells to peripheral tissues such as lung, liver, lymph nodes and bone marrow where the ligand CXCL12 is constitutively produced (Müller et al. 2001).