Perrino CM, et al. – The morphologic findings of tumors within the renal cell carcinoma, unclassified (RCCU) category was elucidated in this research. It also investigated the most frequent morphologic features leading to classification difficulty. It was reported that RCCU included tumors with varied morphologic features and a broad biologic spectrum. It was deduced that the most common source of diagnostic difficulty was tumors composed of predominantly eosinophilic cells.