Test Details: Immunohistochemical testing is carried out on biopsy/resected tissue specimen. This testing helps to identify specific antigens (proteins) within tissue sections utilizing an antigen-specific antibody .Cytokeratin 20 is a type I cytokeratin, an intermediate filament and the main protein of mature enterocytes and goblet cells in gastrointestinal mucosa . Cytokeratin 20 is constantly expressed by adenocarcinoma of the gastrointestinal tract, pancreas, and extrahepatic bile duct system, mucinous ovarian carcinoma, and less frequently transitional cell carcinoma . Also characteristic, is the dot-like perinuclear staining pattern in Merkel cell carcinoma . Cytokeratin 20 is a useful marker to discriminate between reactive atypia and dysplasia of transitional epithelium of the urinary tract.