CXCR4 and breast cancer: Among the cytokine receptor systems, CXCR4, a 7-transmembrane G-protein-coupled receptor for the chemokine ligand CXCL12 (formerly known as Stromal-cell Derived Factor-1alpha, SDF-1α), has been shown to be consistently expressed in human breast cancer cells, and activation of SDF-1α/CXCR4 axis is supposed to be crucial in breast cancer migration and metastasis [9–13].