CCL21 and neoplasm: Moreover, CXCR4 and CCR7 interact with their respective ligands chemokine (C-X-C motif) ligand 12 (CXCL12) and chemokine (C-X-C motif) ligand 21 (CCL21), enabling tumor cells to have “chemotaxis” and promoting their metastasis to the intended organs (23, 58).