MUC5AC and Unclassified Renal Cell Carcinoma: The WHO definition of uRCC (2016 and updated 2022 classifications) includes tumors with mixed histologic features (admixed patterns of recognized subtypes), unclassified oncocytic tumors (that do not meet criteria for chromophobe RCC or oncocytoma), pure sRCC without identifiable epithelial elements, and other uncommon patterns such as those with mucin secretion or dual epithelial-stromal differentiation [3,47].