CK19 is a cytokeratin with a molecular weight of 40 kDa. It is mainly expressed in monolayer epithelium and ductal epithelium, and can be detected in the basal layer of squamous epithelium and epidermis. It is a good marker of epithelial differentiation. It is mainly used for the diagnosis of adenocarcinoma. Hepatocytes do not express CK19, but bile duct epithelium does, so it can be used to distinguish hepatocellular carcinoma from intrahepatic cholangiocarcinoma and metastatic adenocarcinoma. CK19 is partially or weakly expressed in normal thyroid tissue, follicular adenoma, nodular goiter and papillary hyperplasia, but it can be diffusely and strongly expressed in thyroid papillary carcinoma.