MUC5AC and Unclassified Renal Cell Carcinoma: Reported examples of uRCC include tumors with mucinous or signet-ring cell features (mucin production is very uncommon in typical RCC subtypes), tumors with prominent stromal components (mimicking biphasic tumors where epithelial elements are intermixed with spindle cell stroma in a way that does not fit carcinosarcoma or mixed epithelial-stromal tumor categories), or tumors composed of cells that do not resemble any normal renal cell or any well-defined tumor type [44].