CXCL12 and cancer: Its ligand, C-X-C motif chemokine ligand 12 (CXCL-12), is constitutively produced by osteoblasts, fibroblasts, and endothelial cells to retain HSCs within the bone marrow, thus facilitating cancer cells aberrantly overexpressing CXCR-4 to migrate into the bone [31,32].