CXCL12 and cancer: Circulating cancer cells enter the bone through the wide-channeled sinusoids of the bone marrow cavity, invade the marrow stroma, and travel to the endosteal bone surface, in a process mediated by stromal cell-derived factor 1 (SDF1), also known as the C-X-C motif chemokine 12 (CXCL12)—C-X-C chemokine receptor type 4 (CXCR4) pathway [35,36].