CXCR4 and cancer: Metastatic cancer cells are attracted and retained in the bone marrow through the sensing and signaling of chemokines, for example, the C-X-C motif chemokine ligand 12 (CXCL12), which is expressed in bone marrow stromal cells, attracts tumor cells overexpressing the C-X-C chemokine receptor type 4 (CXCR4) (13).