MUC5AC is a glycoprotein with a high molecular weight, recognized as a secreted mucin made by epithelial cells. This substance is mainly found in non-cancerous gastric tissue and healthy respiratory tract tissue. When neoplastic changes occur, the expression of mucins in these tissues can change, resulting in specific patterns of expression. The presence of MUC5AC in the cytoplasm is observable in gastrointestinal adenocarcinomas, particularly in those affecting the esophagus and stomach, as well as in the pancreatobiliary area, which includes cholangiocarcinomas. Additional cancers that may show MUC5AC expression are endometrial and lung carcinomas. MUC5AC can also be present in Paget’s disease.