CXCL12 and cancer: Both CXCR4 and its ligand C-X-C motif chemokine 12 (CXCL12) act as positive regulators of the invasion-metastasis cascade, and their pro-tumor effects have been attributed to i) autocrine mechanisms promoting both proliferation and angiogenesis; and ii) paracrine mechanisms leading to the recruitment of CXCR4+ cancer/immune/stromal cells to CXCL12-overexpressing organs (i.e., bone, liver and lung), resulting in the formation of the so-called pre-metastatic niche [7, 8].