Cathepsin K appears to be more powerful than other commonly used markers in diagnosing a wide spectrum of PEComas and distinguishing them from the majority of human cancers.

We have shown that loss of BRM expression is a common feature among poorly differentiated tumours in clear cell RCCs. We hypothesize that loss of BRM expression is involved in tumor de-differentiation in clear cell RCCs and may play an important role during tumour progression.

Renal cell carcinomas (RCCs) with t(6;11)(p21;q12) are extremely rare and characterized by specific chromosome translocation, involving the transcription factor EB (TFEB). Fewer than 30 cases have been described in the literature. We examined 7 additional cases of this rare tumor by clinicopathologic, immunohistochemical, molecular, and ultrastructural analyses. Four tumors had the typical morphologic features of TFEB RCCs, whereas 3 cases demonstrated uncommon morphologic features, mimicking epithelioid angiomyolipoma, chromophobe cell RCC, and clear cell RCC, respectively. Immunohistochemically, aside from TFEB and cathepsin K, kidney-specific cadherin was another sensitive and relatively specific marker for TFEB RCCs, supporting a distal nephron origin for these renal tumors. We also observed different ultrastructures including mitochondrion with areas of lipofuscin pigment in the smaller cells in these cases. An identical Alpha-TFEB fusion gene, 486 bp, was identified in 2 cases. In addition to the polymerase chain reaction method, we also developed a fluorescence in situ hybridization assay to serve as a cost-effective and time-efficient diagnostic tool. We detected a TFEB gene rearrangement in all 7 cases using the fluorescence in situ hybridization method. TFEB RCC seemed to be an indolent tumor. During a mean follow-up of 31 months, none of the cases developed tumor recurrence, progression, or metastasis.