Cytokeratin 20 (CK20) is an intermediate filament protein with a molecular weight of 46 kDa. Its expression is primarily restricted to specific types of epithelium, including gastric and intestinal epithelium, urothelium, and Merkel cells. CK20 belongs to the Type I cytokeratin family and is a major protein found in mature enterocytes and goblet cells in the gastric and intestinal mucosa.

The expression of CK20 antibody is observed in several types of adenocarcinomas, such as those affecting the colon, stomach, pancreas, and biliary system. It is also expressed in mucinous ovarian tumors, transitional-cell carcinomas of the urinary tract, and Merkel cell carcinomas. CK20 is particularly useful in differentiating specific types of simple epithelial cells in the urinary tract and distinguishing between normal and malignantly transformed epithelia.
However, CK20 antibody shows minimal reactivity in squamous cell carcinomas and adenocarcinomas of the breast, lung, and endometrium. It is also non-reactive in non-mucinous tumors of the ovary and CK20ll-cell carcinomas. To differentiate colon carcinomas (CK20+) from ovarian, pulmonary, and breast carcinomas, CK20 antibody is often used in combination with CK7 and other antibodies.