CXCL12 and melanoma: The C-C chemokine receptor type 7 (CCR7) promotes melanoma cell metastasis to the C-C motif ligand 21 (CCL21)-rich lymph nodes while the C-X-C chemokine receptor type 4 (CXCR4)/C-X-C motif chemokine ligand 12 (CXCL12) axis facilitates pulmonary metastasis [17,18].