CA9 and nonpapillary renal cell carcinoma: These included cytokeratin 18 (CK18), an intermediate filament protein expressed in epithelial tissues and carcinomas, which is known to have a diffuse, strong staining pattern in ccRCC tumors (33), vimentin, a major constituent of intermediate filaments known to be highly expressed in ccRCC tumors (31), and CAIX, a hypoxia-responsive gene highly expressed in ccRCC tumors (39).