CK20 is a cytokeratin with a molecular weight of 46 kDa, which is mainly expressed in gastrointestinal epithelium, urethral epithelium and Merkel cells. It is mainly used for the diagnosis of gastrointestinal adenocarcinoma, ovarian mucinous tumor, Merkel cell carcinoma and cholangiopancreatic adenocarcinoma. CK20 was negative in squamous cell carcinoma, lung adenocarcinoma, endometrial adenocarcinoma, breast cancer, small cell lung cancer and ovarian non mucinous tumor. It is often used in combination with CK7 and villin to judge the tissue source of adenocarcinoma.