CXCR4 and lung cancer: C-X-C motif chemokine ligand 12 (CXCL12), also known as the sole ligand of CXCR4, has been verified to be expressed in many tissues and cell types [40] Previous studies demonstrated that the positive expression rate of CXCL12 in lung cancer was 31.3–80% [14, 17, 27, 34].