CXCR4 and head and neck squamous cell carcinoma: With respect to CXCR4, dormant/non-proliferative/slow-cycling disseminated tumor cells in bone marrow highly express TGF-β2 to maintain SDF-1-CXCR4 overexpression and inhibition of SDF-1-CXCR4 signaling by down-regulating TGF-β2 reversed the drug resistance of HNSCC cell line HEp3 via reactivation of cell proliferation [29].