CXCR4 and breast carcinoma: For example, elevated expression of transforming growth factor (TGF-α) in renal cell carcinoma may provide an autocrine stimulation for RCC proliferation [37]; CXCR4 is involved in the progression and metastasis of renal cell carcinoma [38] and breast cancers [39]; and HIF-2α, another inducible member of the HIF family, has been shown to be important for the malignant transformation of renal cell carcinoma [40].