CXCL12 and breast carcinoma: The transmembrane G protein-coupled receptor CXCR4 is the most prominent chemokine receptor expressed in breast cancer cells (but not in normal breast cells) and represent a major factor in breast cancer metastasis due to migration of the cancerous cells through signaling by its unique ligand (i.e., CXCL12) to surrounding tissues [67, 69–71].