CXCR4 and breast cancer: The chemokine CXCL12, also known as SDF-1, binds to G-protein-coupled seven-transmembrane-domain chemokine receptor 4 (CXCR4), activating signaling pathways (PI-3 K/AKT, ERK1/2 and MAPK) and controlling cell survival, migration and proliferation, with the CXCR4/CXCL12 axis showing a key role in breast cancer progression and in many other cancers, as in liver, lung, bone, brain, prostate, ovarian, cervical, colorectal and pancreatic tumors [1–4].