Cytokeratin 20 is a type I keratin which is primarily expressed in gastric and intestinal epithelium, urothelium, and Merkel-cells. The keratins are intermediate filament proteins responsible for the structural integrity of epithelial cells and are subdivided into cytokeratins and hair keratins. The type I cytokeratins consist of acidic proteins which are arranged in pairs of heterotypic keratin chains. Their genes are clustered in a region of chromosome 17q12-q21. Cytokeratin 20 is a major cellular protein of mature enterocytes and goblet cells and is specifically expressed in the gastric and intestinal mucosa. It is also expressed in adenocarcinomas of the colon, stomach, pancreas and the bile system and is present in mucinous ovarian tumors, transitional-cell and Merkel-cell carcinomas. Notably, the squamous cell carcinomas and adenocarcinomas of the breast, lung, and endometrium, non-mucinous tumors of the ovary, and small cell carcinomas lack cytokeratin 20.