CXCR4 and cancer: The CXC chemokine receptor 4 (CXCR4) interacts specifically with its chemokine ligand, stromal cell-derived factor 1 (SDF-1/CXCL12), to exert proliferative and chemotactic effects in CXCR4-expressing cancer cells, accounting for an association of CXCR4 with metastasis in numerous human tumors, including lung, breast, prostate, colon, and thyroid cancer [15–19].