Cytokeratin 20 is a 46 kDa intermediate filament protein that reacts primarily with gastric and intestinal epithelium, urothelium and Merkel cells. Anti-cytokeratin 20 is useful in identifying specific types of these epithelial cells under normal, hyperplastic and neoplastic conditions. Cytokeratin 20 has been detected in various adenocarcinomas, including those of the colon and stomach. Merkel cell carcinomas have shown reactivity. In contrast, carcinomas of the breast are generally non-reactive.