Keratin 20 / cytokeratin20 (CK20) is a Type-I keratin which is primarily expressed in gastric and intestinal epithelium, urothelium, and Merkel-cells. CK20 is expressed in adenocarcinomas of the colon, stomach, pancreas and the bile system. CK20 is also 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 in CK20.