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Cytokeratins are proteins of cytoskeletal intermediate filaments, and their main function is to enable cells to withstand mechanical stress. In humans, 20 different cytokeratin isotypes have been identified. Cytokeratins 8, 18, 19, and 20 have been associated with bladder. Cytokeratin 18 （KRT18, also called K18）, found in epithelial cells, is released from hepatocytes upon death.