CXCR4 and renal cell carcinoma: In an effort to address the potential role of CXCR4 as a biomarker for predicting the metastatic potential of RCC, we first measured CXCR4 expression on circulating cytokeratin+ cells in patients with mRCC, and found significantly increased levels of cytokeratin+ cells that co-expressed CXCR4, as compared to normal human subjects.