CK20 (KRT20) is an intermediate filament keratin that is expressed in epithelial cells in enterocytes and goblet cells of the gastrointestinal mucosa, in some endocrine cells and Merkel cells of the epidermis. It is frequently used as a differentiation marker in the colon, and is used in conjunction with CK7 to distinguish colon carcinomas (CK20 +, CK7 -) from ovarian, pulmonary, or breast cancers. Poorly differentiated colorectal tumors and those with high rates of MSI may also stain negatively for this marker. It is also positive in adenocarcinomas of the gallbladder and bile ducts, ductal cell adenocarcinomas of the pancreas, cholangiocarcinomas, mucinous ovarian tumors, transitional-cell carcinomas, and Merkel-cell carcinomas of the skin. CK20 is usually negative in squamous cell carcinomas, breast cancers, lung adenocarcinomas, endometrial cancer, and small cell carcinomas. Staining for this target is expected to be cytoplasmic. References: Proc Natl Acad Sci U S A. 2009 Feb 10;106(6):1936-41, PMID: 19188603; Histology and Histopathology 27 (3):347–56. 2012/03, PMID: 22237712; Cell Stem Cell 8 (5):511–24. 5/6/2011, DOI: 10.1016/j.stem.2011.02.020;