CXCR4 and central nervous system cancer: Transforming growth factor-β (TGF-β) plays important functions in regulating GBM progression and self-renewal 13, 16, 28, high TGF-β activity confers poor prognosis in glioma patients 29, and the oncogenic MSH6-CXCR4-TGF-β1 feedback loop is a novel therapeutic target for GBM 30.