CXCR4 and breast cancer: Particularly, CAFs (Cancer-Associated Fibroblasts), the most abundant cell type in breast cancer stroma, produce a plethora of growth factors, extracellular matrix proteins and chemokines, among which SDF-1α, via its cognate receptor CXCR4, acts through autocrine- and paracrine-signaling mechanisms to support tumor progression [19–22].