CXCL12 and cancer: Chemokines have a critical role in cancer progression, and two chemokine ligand–receptor systems are common key mediators of tumor cell metastasis for several malignancies; the CXCL12–CXCR4 axis facilitates metastasis to distant organs, whereas the CCL21–CCR7 chemokine ligand–receptor pair favors metastasis to lymph nodes [82].