We have recently isolated a gene, Ankyrin-repeated protein with a proline-rich region (ARPP), that is highly expressed in the skeletal and cardiac muscle. Our previous immunohistochemical analysis revealed that ARPP expression was augmented in rhabdomyosarcoma but scarcely detectable in leiomyosarcoma, showing that ARPP is a useful marker for rhabdomyosarcoma. In the present study, we generated the anti-ARPP monoclonal antibody, YAS11, immunoreactive with the N-terminal region (amino-acids residues 1-145) of the ARPP protein. Further, we immunohistochemically analyzed 100 renal tumors including 14 oncocytomas, and 86 renal cell carcinomas (RCCs). We found that ARPP was highly expressed in 12 of the 14 (85.7%) oncocytomas, but was detectable in only four of the 86 (4.7%) RCCs. Interestingly, ARPP was not detected in any of 11 chromophobe RCCs, suggesting that ARPP may be useful for differential diagnosis between oncocytoma and chromophobe RCC. Furthermore, we found that ARPP was selectively expressed in part of the distal renal tubule in normal kidney. Immunoelectron microscopy with anti-ARPP antibody revealed that ARPP was localized in mitochondria and nuclei in both the normal distal renal tubule and oncocytoma, suggesting that oncocytoma may be derived from the distal nephron, and probably from part of the distal renal tubule.