CXCL12 and cancer: MSCs are thought to contribute to CSC niche generation, thus regulating cancer cell stemness through multiple pathways and secreted factors (i.e., IL-6 and CXCL7 [193], PGE-2 [194], EGF, bFGF, bone morphogenic protein (BMP) 4, TGF-β1, SDF-1α, and CCL5 [195], among others) that increase CSC self-renewal and expand the CSC population.