CXCR4 and breast carcinoma: The bone tropism of breast cancer cells is in part mediated by chemokines and their receptors, exemplified by C-X-C chemokine receptor type 4 (CXCR4) expressed as one of the signature genes on bone-seeking breast cancer cells and its ligand stromal-derived-factor-1 expressed by cells within the bone environment including osteoblasts [2, 3].