Immunohistochemical analysis of monoclonal antibodies to renal antigens. Application in the diagnosis of renal cell carcinoma.

Four mouse monoclonal antibodies reacting with renal proximal epithelium were characterized on normal adult tissues, fetal kidneys, and various malignancies. These four antibodies were found to react with renal cell carcinomas (RCCs) but not with a variety of other tumors. Three antibodies react only with some primary RCCs and not with metastatic RCCs. The fourth antibody, RC 38, reacts with 46 of 47 primary RCCs tested and with 8 of 13 RCC metastases. No reaction was seen with 179 tumors of various origin, which indicates that this antibody is very useful in diagnosing RCC.     

Expression of pyruvate kinase M2 in preneoplastic hepatic foci of N-nitrosomorpholine-treated rats

 The expression of the pyruvate kinase (PK) isoenzymes L and M₂ was analysed in the livers of rats treated with the hepatocarcinogenic agent N-nitrosomorpholine (NNM) in the drinking water. In control animals L-PK expression was restricted to liver parenchymal cells, whereas M₂-PK was detected in bile duct epithelial, blood vessel wall, endothelial and Kupffer cells. In rats treated with NNM proliferating oval cells were consistently L-PK negative and M₂-PK positive, while the ductal cells of cholangiofibroses were clearly L-PK positive and coexpressed M₂-PK. However, no morphological differentiation of ductal cells into hepatocyte-like cells was observed. In the clear and acidophilic cell foci storing glycogen in excess strong staining for L-PK was observed. In glycogen-poor foci induced by NNM a shift from L-PK to M₂-PK expression takes place. Received: 24 March 1998 / Accepted: 13 November 1998     

The diagnostic utility of MOC31, BerEP4, RCC marker and CD10 in the classification of renal cell carcinoma and renal oncocytoma: an immunohistochemical analysis of 328 cases

AIMS: To demonstrate the diagnostic utility of MOC31, BerEP4, renal cell carcinoma marker (RCC Ma) and CD10 in the classification of RCC and renal oncocytoma, based upon a comprehensive immunohistochemical analysis. METHODS AND RESULTS: Immunohistochemistry was performed on 328 samples consisting of 256 clear cell/conventional, 27 papillary, 28 chromophobe, five collecting duct, five unclassified RCCs and seven renal oncocytomas using antibodies MOC31, BerEP4 and antibodies against cytokeratins (KL-1, CAM5.2, 34betaE12, cytokeratin 7), RCC Ma, epithelial membrane antigen, E-cadherin, CD10, CD15 and vimentin. Multivariate analysis showed that MOC31, BerEP4, RCC Ma and CD10 have discriminatory value. MOC31 and BerEP4 chiefly labelled distal tubules of normal kidney while RCC Ma and CD10 labelled the proximal tubules. Twenty-three chromophobe RCCs (82%) were reactive for MOC31, while only four clear cell RCCs and three papillary RCCs were positive for this marker. Clear cell RCCs were characterized by a high positive rate for CD10 (82%) and a low positive rate for BerEP4 (27%). Papillary RCCs frequently coexpressed RCC Ma and BerEP4 (51%). All renal oncocytomas were negative for MOC31 and CD10. CONCLUSIONS: MOC31 has diagnostic merit in discerning chromophobe RCC. The CD10+/BerEP4- profile and RCC Ma+/BerEP4+ profile achieve moderate sensitivity and good specificity for clear cell RCC and papillary RCC, respectively. The non-reactivity for both MOC31 and CD10 is helpful in distinguishing renal oncocytoma from RCC. When properly selected, antibodies have immunohistochemical diagnostic utility for the classification of renal cortical epithelial tumours.     

Renal cell carcinoma marker reliably discriminates central nervous system haemangioblastoma from brain metastases of renal cell carcinoma

Aims:  The distinction between central nervous system (CNS) metastases of clear cell renal cell carcinoma (RCC) and CNS haemangioblastoma still poses a challenge to the pathologist. Since both entities occur in von Hippel–Lindau disease, this aggravates the issue. The antibody renal cell carcinoma marker (RCC-ma) has been suggested to identify primary RCCs specifically, but its value for diagnosing metastases of RCC is controversial. The aim was to assess two distinct clones of the RCC-ma for their potential to: (i) identify primary RCCs and (ii) differentiate between CNS metastases of clear cell RCC and CNS haemangioblastomas.
Methods and results:  Using tissue microarrays, 77% ( n  = 363; PN-15) and 66% ( n  = 355; 66.4C2) of clear cell RCCs, and 93% (PN-15) and 74% (66.4C2) of papillary RCCs ( n  = 46) were immunopositive for RCC-ma, whereas none of the investigated chromophobe RCCs ( n  = 22) or any of the oncocytomas ( n  = 15) showed immunoreactivity. Importantly, 50.9% of CNS metastases of clear cell RCCs ( n  = 55) exhibited RCC-ma expression, whereas all CNS haemangioblastomas (71) were negative.
Conclusions:  Both RCC-ma clones, despite some variation in their sensitivity to detect clear cell and papillary RCCs, are of value in differentiating subtypes of primary RCC and are excellent markers for discriminating clear cell lesions in the brain.     

Expression of serine protease matriptase in renal cell carcinoma: correlation of tissue microarray immunohistochemical expression analysis results with clinicopathological parameters

Serine protease matriptase (matriptase) cleaves and activates proteins implicated in the progression of cancer and represents a potential therapeutic target. Immunohistochemical analysis of matriptase was performed in tissue microarrays of 168 renal cell carcinomas (RCCs). All subtypes of RCC showed significant immunohistochemical expression of matriptase. In contrast, no expression occurred in areas of RCC with sarcomatous differentiation (SRCC) and in normal collecting tubules. The matriptase scores were significantly higher in papillary RCC (341+/-28) and clear cell RCC with granular cell differentiation (GRCC; 324+/-27) than in other histologic subtypes of RCC. In GRCC, matriptase scores were correlated with TNM staging and nuclear grading. Matriptase was overexpressed in all subtypes of RCC, and matriptase scores could distinguish between conventional clear cell RCC, GRCC, and SRCC.     

New and emerging renal tumour entities

In this review, we discuss new and emerging renal cell carcinoma (RCC) entities, including anaplastic lymphoma kinase (ALK) RCC, oncocytic variant of chromophobe RCC, atrophic kidney-like renal tumour, biphasic alveolosquamoid RCC, tubulocystic RCC, thyroid-like follicular carcinoma of the kidney, succinate dehydrogenase-deficient RCC, Birt–Hogg–Dubé syndrome-associated renal tumour, hereditary leiomyomatosis/renal cell carcinoma associated RCC, tuberous sclerosis-associated RCC, PTEN hamartoma tumour syndrome, clear cell papillary RCC, acquired cystic disease-associated RCC, Xp11.2 RCC, t(6;11) RCC and renal hemangioblastoma. These tumours have clinical, pathological and genetic features distinct from other common RCCs and therefore are important to recognize. Some of them have been recognized as distinct histological subtypes in the 2016 World Health Organization Classification. However, further studies are needed to elucidate their clinicopathologic features and molecular mechanisms.     

Genetic and epigenetic alterations during renal carcinogenesis

Renal cell carcinoma (RCC) is not a single entity, but comprises a group of tumors including clear cell RCC, papillary RCC and chromophobe RCC, which arise from the epithelium of renal tubules. The majority of clear cell RCCs, the major histological subtype, have genetic or epigenetic inactivation of the von Hippel-Lindau (VHL) gene. Germline mutations in the MET and fumarate hydratase (FH) genes lead to the development of type 1 and type 2 papillary RCCs, respectively, and such mutations of either the TSC1 or TSC2 gene increase the risk of RCC. Genome-wide copy number alteration analysis has suggested that loss of chromosome 3p and gain of chromosomes 5q and 7 may be copy number aberrations indispensable for the development of clear cell RCC. When chromosome 1p, 4, 9, 13q or 14q is also lost, more clinicopathologically aggressive clear cell RCC may develop. Since renal carcinogenesis is associated with neither chronic inflammation nor persistent viral infection, and hardly any histological change is evident in corresponding non-tumorous renal tissue from patients with renal tumors, precancerous conditions in the kidney have been rarely described. However, regional DNA hypermethylation on C-type CpG islands has already accumulated in such non-cancerous renal tissues, suggesting that, from the viewpoint of altered DNA methylation, the presence of precancerous conditions can be recognized even in the kidney. Genome-wide DNA methylation profiles in precancerous conditions are basically inherited by the corresponding clear cell RCCs developing in individual patients: DNA methylation alterations at the precancerous stage may further predispose renal tissue to epigenetic and genetic alterations, generate more malignant cancers, and even determine patient outcome. The list of tumor-related genes silenced by DNA hypermethylation has recently been increasing. Genetic and epigenetic profiling provides an optimal means of prognostication for patients with RCCs. Recently developed high-throughput technologies for genetic and epigenetic analyses will further accelerate the identification of key molecules for use in the prevention, diagnosis and therapy of RCCs.     

Natural history of the Nihon (Bhd gene mutant) rat, a novel model for human Birt–Hogg–Dubé syndrome

In the Nihon rat, an established model of hereditary renal cell carcinoma (RCC), the propensity for tumor development, is inherited as an autosomal dominant trait due to a single germline nucleotide insertion mutation in the rat Bhd ortholog. The Birt–Hogg–Dubé (BHD) syndrome is a rare autosomal dominant disease characterized by fibrofolliculoma, pulmonary cysts, spontaneous pneumothorax, and renal neoplasm. The renal lesions of the Nihon rat are characterized, and extrarenal lesions are also described in this work. The earliest lesion of the RCC was identified as an altered tubule at as early as 3 weeks of age and rapidly progressed through adenoma to carcinoma with the primary cell type being clear/acidophilic where some similarities were evident to RCCs in BHD syndrome. The Nihon rats demonstrate a heterotopic ossification within RCCs and three extrarenal lesions, clear cell hyperplasia/adenoma of the endometrium, clear cell change of the epithelium of striated portions of salivary glands, and cardiac rhabdomyomatosis. This rat model of hereditary RCC provides a useful tool for analyzing the series of events leading to renal tumorigenesis and for studying BHD gene functions.     

High endogenous avidin binding activity: an inexpensive and readily available marker for the differential diagnosis of kidney neoplasms

It has been documented that some tissues, such as salivary gland, liver, cardiac and skeletal muscles and kidney, have high level endogenous biotin or endogenous avidin binding activity (EABA). Limited data is available on EABA in renal cell neoplasms. A tissue microarray (TMA) was constructed that included oncocytoma (n=30), chromophobe renal cell carcinoma (RCC) (n=18), clear cell RCC (n=45), clear cell RCC with granular/eosinophilic (G/E) features (n=19), papillary RCC (n=21), papillary RCC with G/E features (n=29) and benign renal tissues (n=31). The TMA slides were stained with or without biotin blocker and analyzed using the automated cellular imaging system (ACIS(R)). Without biotin blocker, a high positive rate of EABA was found in oncocytoma (56/60, 93%) and normal renal tubules (46/60, 77%). A moderate positive rate of EABA was found in clear cell and papillary RCCs with G/E features (13/39, 33% and 19/55, 35%, respectively). Chromophobe RCC and RCC without G/E features had essentially no EABA. With biotin blocker, benign renal tissue and clear cell RCC were negative for EABA; but a significant number of renal oncocytoma (29/60, 48%) and a few papillary RCC with G/E features (5/52, 10%) remained positive for EABA. In conclusion, high EABA may be used to differentiate oncocytoma from chromophobe RCC, and the staining results must be interpreted with caution when avidin-biotin detection system is used in diagnosing renal neoplasms.     

Genetic and epigenetic alterations during renal carcinogenesis

Renal cell carcinoma (RCC) is not a single entity, but comprises a group of tumors including clear cell RCC, papillary RCC and chromophobe RCC, which arise from the epithelium of renal tubules. The majority of clear cell RCCs, the major histological subtype, have genetic or epigenetic inactivation of the von Hippel-Lindau (VHL) gene. Germline mutations in the MET and fumarate hydratase (FH) genes lead to the development of type 1 and type 2 papillary RCCs, respectively, and such mutations of either the TSC1 or TSC2 gene increase the risk of RCC. Genome-wide copy number alteration analysis has suggested that loss of chromosome 3p and gain of chromosomes 5q and 7 may be copy number aberrations indispensable for the development of clear cell RCC. When chromosome 1p, 4, 9, 13q or 14q is also lost, more clinicopathologically aggressive clear cell RCC may develop. Since renal carcinogenesis is associated with neither chronic inflammation nor persistent viral infection, and hardly any histological change is evident in corresponding non-tumorous renal tissue from patients with renal tumors, precancerous conditions in the kidney have been rarely described. However, regional DNA hypermethylation on C-type CpG islands has already accumulated in such non-cancerous renal tissues, suggesting that, from the viewpoint of altered DNA methylation, the presence of precancerous conditions can be recognized even in the kidney. Genome-wide DNA methylation profiles in precancerous conditions are basically inherited by the corresponding clear cell RCCs developing in individual patients: DNA methylation alterations at the precancerous stage may further predispose renal tissue to epigenetic and genetic alterations, generate more malignant cancers, and even determine patient outcome. The list of tumor-related genes silenced by DNA hypermethylation has recently been increasing. Genetic and epigenetic profiling provides an optimal means of prognostication for patients with RCCs. Recently developed high-throughput technologies for genetic and epigenetic analyses will further accelerate the identification of key molecules for use in the prevention, diagnosis and therapy of RCCs.     

The tumour suppressor gene CEACAM1 is completely but reversibly downregulated in renal cell carcinoma

CEACAM1 acts as a tumour suppressor in various epithelial tumours. On the other hand, de novo expression of CEACAM1 is strongly associated with reduced disease-free survival of melanoma and non-small cell lung carcinoma patients. Since effector functions of natural killer and T cells are inhibited by homophilic CEACAM1 interaction, immune escape could be responsible for the poor prognosis of these patients. Here, we describe CEACAM1 expression in normal kidney, renal adenomas and renal cell carcinomas (RCC) using a novel antibody generated by genetic immunization. In normal kidney, CEACAM1 was found in epithelial cells of proximal tubules and in endothelial cells. In contrast, tumour cells of 30 clear cell, three chromophobic, and two chromophilic RCCs were completely devoid of CEACAM1. Renal adenomas also lacked CEACAM1 expression. Similarly, RCC cell lines CaKi1, CaKi2, A498, and RCC26 exhibited no or low-level CEACAM1 expression. However, CEACAM1 expression was transiently induced in A498 cells upon contact with allogeneic CD8+ T cells, mediated at least in part by interferon-gamma. Furthermore, the majority of tumour-infiltrating T and NK cells expressed CEACAM1 upon stimulation. Thus, transient expression of the tumour suppressor CEACAM1 by tumour cells and subsequent homophilic interaction with CEACAM1 on tumour-infiltrating lymphocytes could represent a novel immune escape mechanism in RCC.     

Are heterogenous results of EGFR immunoreactivity in renal cell carcinoma related to non-standardised criteria for staining evaluation?

AIMS: To assess whether heterogeneity of epidermal growth factor receptor (EGFR) immunoreactivity in renal cell carcinoma (RCC) is related to non-standardised criteria for staining evaluation. METHODS: EGFR expression was investigated in 132 primary and 55 metastatic conventional RCCs using a tissue microarray technique. RESULTS: Overall, membranous and/or cytoplasmic EGFR immunostaining was present in 123 of 132 (93%) primary and 49 of 53 (92%) metastatic RCCs, with extensive immunoreactivity (> 50% of tumour cells) in 110 of 132 (83%) primary tumours and 39 of 53 (73%) metastases. Cytoplasmic staining was associated with high tumour stage and high tumour grade. In addition, strong membranous staining (score 3+) prevailed in high grade RCCs. Cytoplasmic immunostaining was associated with an unfavourable prognosis, whereas overall (cytoplasmic and membranous) immunoreactivity and intensity of membranous staining were not. CONCLUSIONS: Different methods of immunohistochemical evaluation led to different results, strengthening the need for standardisation, especially against a background of rapidly evolving EGFR targeted cancer treatment strategies.     

Immunohistochemical detection of P504S in primary and metastatic renal cell carcinomas.

P504S/alpha-methylacryl CoA racemase has been shown to be a relatively sensitive and specific positive marker for prostatic adenocarcinoma. The potential utility of P504S in renal cell neoplasms has not been explored in a large series. We assessed the diagnostic value of P504S in 332 cases of nonprostatic neoplasms using the avidin-biotin-peroxidase complex technique, including 115 renal neoplasms, 28 metastatic renal cell carcinomas (RCCs), and 189 nonrenal neoplasms. The results demonstrated that a granular, cytoplasmic staining pattern for P504S was observed in 48 of 70 (68.6%) conventional (clear cell) RCCs, 15 of 15 (100%) papillary RCCs, 2 of 7 (29%) chromophobe RCCs, and 2 of 8 (25%) oncocytomas. Among the 70 cases of clear cell RCC, positivity of P504S was seen in 40%, 71%, 94%, and 75% of RCCs with Furhman nuclear grade I, II, III, and IV, respectively. Strong immunostaining was present in each case (86/86) in the proximal tubules adjacent to the renal neoplasm. Eighty-two percent of metastatic RCCs (23/28) were positive for P504S. However, only 24 of 189 (13%) nonrenal malignancies were positive. The 24 positive cases included 12 of 13 (92%) colorectal adenocarcinomas, 6 of 30 (20%) ductal carcinomas of the breast, and 4 of 23 (17%) adenocarcinomas of the lung. These findings suggest that P504S is a useful marker in diagnosing primary and metastatic RCCs, although it has little value in differentiating chromophobe RCC from oncocytoma.     

Expression of kidney-specific cadherin in chromophobe renal cell carcinoma and renal oncocytoma

Kidney-specific cadherin (Ksp-cad) recently was proposed to differentiate chromophobe renal cell carcinoma (RCC) from oncocytoma based on a finding of Ksp-cad expression in 97% of chromophobe RCCs but only 3% of oncocytomas. However, another study showed no difference in Ksp-cad immunoreactivity between these 2 tumors. We attempted to evaluate Ksp-cad expression in renal tumors using expression microarrays and immunohistochemical analysis. Ksp-cad messenger RNA (mRNA) levels were examined in 158 renal tumors, including 15 chromophobe RCCs and 15 oncocytomas. Immunohistochemical analysis was performed on tissue microarrays containing 125 renal tumors, including 36 chromophobe RCCs and 41 oncocytomas. Ksp-cad mRNA compared with normal kidney tissue was 89% in chromophobe RCC and 64% in oncocytoma. Furthermore, 31 of 36 chromophobe RCCs and 31 of 41 oncocytomas showed Ksp-cad immunoreactivity. Ksp-cad was present in chromophobe RCCs and oncocytomas at mRNA and protein levels, providing strong evidence that Ksp-cad immunohistochemical analysis cannot be used in differentiating these tumors.     

C-kit expression in renal oncocytomas and chromophobe renal cell carcinomas

C- kit encodes the membrane-bound tyrosine kinase KIT, whose expression has been identified in several types of human neoplasms. Recently, KIT has been reported to be a marker for chromophobe renal cell carcinoma (RCC) and renal angiomyolipoma. However, expression of this molecule has not been adequately studied in other renal tumors, particularly oncocytoma, which may morphologically resemble chromophobe RCC. In this study, we analyzed c- kit messenger RNA (mRNA) levels in 17 chromophobe RCCs and 20 renal oncocytomas obtained from complementary DNA (cDNA) microarrays. Furthermore, comprehensive immunohistochemical analysis of KIT protein using a monoclonal antibody was performed in 226 renal tumors including chromophobe RCC (n=40), oncocytoma (n=41), clear-cell RCC (n=40), renal angiomyolipoma (n=29), and papillary RCC (n=21) on tissue microarrays (TMAs) and was compared with immunostaining results from 25 chromophobe RCCs and 30 oncocytomas using standard sections. The staining intensity was semiquantitatively graded on a 3-tier scoring system. All chromophobe RCCs and oncocytomas showed significant overexpression of c- kit mRNA. The average increase of mRNA compared with normal kidney tissue was 7.4-fold for chromophobe RCCs and 7.4-fold for oncocytomas. Immunohistochemical expression of KIT was found in most chromophobe RCCs (95% in TMAs and 96% in conventional sections) and oncocytomas (88% in TMAs and 100% in conventional sections) but was infrequently observed in renal angiomyolipomas (17%), papillary RCCs (5%), and clear-cell RCCs (3%). Furthermore, the average KIT immunoreactivity in TMAs was stronger in chromophobe RCC (1.93) and oncocytoma (2.07) than in other subtypes of renal tumors tested, including angiomyolipomas (0.17), papillary RCCs (0.05), and clear-cell RCCs (0.03). In conclusion, we found a significant elevation of c- kit mRNA by cDNA expression microarrays and overexpression of KIT protein by immunohistochemistry not only in chromophobe RCCs but also in oncocytomas. In contrast, immunohistochemical expression of KIT was not detected in most other types of renal cell tumors evaluated. The differential expression of c- kit in these renal tumors may have diagnostic and therapeutic implications.     

Renal cell carcinoma and the renal sinus

Renal sinus fat invasion was incorporated as one of the parameters for pT stage definition of renal cell carcinoma (RCC) only in the latest 2002 American Joint Committee on Cancer/tumor-node-metastasis staging protocol. The current pT3a subcategory (in addition to adrenal gland involvement) groups 2 modes of extrarenal extension by RCC, either by peripheral perinephric fat extension or by renal sinus fat invasion. Recent prospective studies have shown that with more directed gross sampling and histologic evaluation, renal sinus invasion is actually more commonly diagnosed than previously reported, or when compared with retrospectively sampled RCC nephrectomy specimens. These studies have demonstrated that renal sinus invasion is the principal pathway for extrarenal extension for clear cell RCC; the incidence of which is related to size (tumors greater than 4 cm more frequently involve the renal sinus). More significantly, a recent retrospective study of pT3a clear cell RCC nephrectomy specimens showed that tumors invading the renal sinus fat portend a more aggressive outcome than tumors invading only the peripheral perinephric fat. Clear cell RCCs invading the renal sinus are more likely to have higher nuclear grade, regional lymph node involvement and sarcomatoid transformation than tumors invading only the perinephric fat. Given the importance of renal sinus invasion, sampling strategies for nephrectomy specimens should be modified to focus in this region as appropriate and pathologists should be familiar with the histologic criteria for staging renal sinus invasion.     

Immunoexpression of SDHB,FH, and CK20 among eosinophilic renal tumors: A tissue microarray study

BackgroundDifferential diagnosis can be a challenge for eosinophilic subtypes of renal cell tumors due to their overlapping histomorphological and immunohistochemical features. We aimed to investigate the frequency of rare variants of renal cell carcinomas (RCCs) such as succinate dehydrogenase-deficient RCC (SDDRCC), hereditary leiomyomatosis and RCC (HLRCC)-associated RCC, and eosinophilic, solid, and cystic RCC (ESCRCC) in our population.Materials and methodsRenal tumors which could be considered in the eosinophilic tumor category were included: 91 conventional clear cell RCCs with eosinophilic cytoplasm, 72 papillary RCCs, 74 chromophobe RCCs, 88 oncocytomas, and 37 other rare subtypes. Using the tissue microarray method, succinate dehydrogenase B (SDHB), fumarate hydratase (FH), and cytokeratin 20 (CK20) antibodies were performed by immunohistochemistry. Immunohistochemistry was repeated on whole block sections for selected cases. The utility of these antibodies in the differential diagnosis was also investigated.ResultsLoss of SDHB expression was detected in three tumors, two of which showed typical morphology for SDDRCC. In additional two tumors, SDHB showed weak cytoplasmic expression without a mitochondrial pattern (possible-SDHB deficient). None of the tumors showed loss of FH expression. Heterogeneous reactions were observed with SDHB and FH antibodies. Only one ESCRCC was detected with diffuse CK20 positivity.ConclusionSDDRCCs, HLRCC-associated RCCs, and ESCRCCs are very rare tumors depending on the population. Possible weak staining and focal loss of SDHB and FH expression should be kept in mind and genetic testing must be included for equivocal results.     

Expression of S-100 protein in renal cell neoplasms

Polyclonal antibody to S-100 protein has been routinely applied for initial screening of various types of tumors, including, melanocytic tumors and neurogenic tumors. S-100 protein has been shown to have a broad distribution in human tissues, including renal tubules. The potential utility of S-100 protein in renal cell neoplasms has not been extensively investigated. Using an EnVision-Horseradish Peroxidase (HRP; Dako, Carpinteria, Calif) kit, we evaluated the diagnostic value of S-100 protein on tissue microarray sections from 175 cases of renal epithelial neoplasm (145 primary renal neoplasms and 30 metastatic renal cell carcinomas) and 24 non-neoplastic renal tissues. Immunohistochemical stains for pancytokeratin, HMB-45, and Mart-1 were also performed. Western blot using the same antibody (anti-S-100 protein) was performed on 10 cases of renal cell neoplasm. The results demonstrated that nuclear and cytoplasmic staining pattern for S-100 protein was observed in 56 (69%) of 81 conventional (clear cell) renal cell carcinomas (RCCs), 10 (30%) of 33 papillary RCCs, 1 (6%) of 16 ChRCCs, and 13 (87%) of 15 oncocytomas. Among the 81 cases of CRCC, positivity for S-100 protein was seen in 41 (71%) of 58 and 15 (65%) of 23 cases with Furhman nuclear grade I/II and III/IV, respectively. Focal immunostaining was present in 22 (92%) of 24 normal renal tubules. Similar staining pattern was observed in 21 (70%) of 30 metastatic RCCs. Western blotting demonstrated the S-100 protein expression in both renal cell neoplasm and normal renal tissue. Overexpression of S-100 in oncocytomas compared with ChRCCs was confirmed by the data of Western blot and cDNA microarray analysis. Importantly, 14.8% (12/81) of clear cell RCC and 13.3% (4/30) of metastatic RCC revealed an immunostaining profile of pancytokeratin (-)/S-100 protein (+). These data indicate that caution should be taken in interpreting an unknown primary with S-100 positivity and cytokeratin negativity. In addition, it suggests that S-100 has a diagnostic value in differentiating oncocytoma from ChRCC.