CA9 and renal cell carcinoma: Specific targeting molecules, most of which are proteins, have been proposed (C-reactive protein (CRP), PTEN, carbonic anhydrase IX (CAIX), hypoxia-inducible factors (HIF-1α and HIF-1β), vascular endothelial growth factor (VEGF, CD44, E-cadherin, osteopontin, antigen Ki-67 and tumor protein p53), and monitoring their activity might generate a timely prognosis of the metastatic potential of RCC.