CXCR4 and renal cell carcinoma: Having established that CXCL12 depletion significantly impaired the metastasis of RCC to specific target organs, we next wanted to determine whether this effect was mediated by CXCL12/CXCR4 biological axis on RCC metastases, or by alterations of RCC tumor or tumor cell biology, such as tumor-associated vascular density, or tumor cell proliferation and/or apoptosis.