Mucin is a high M.W. (>1,000 kDa) glycoprotein, expressed by mucus cells of the gastric epithelium and by goblet cells of the fetal, precancerous and cancerous colon, but not by those of the normal colon. It also appears in other epithelial tissues, which are embryologically derived from the foregut (epigastric and bronchial epithelium) and in Müller ducts (mucus cells of the endocervix and urethral epithelium near the prostatic utriculus). The expression of MUC5AC has been frequently observed in gastric carcinomas and may be used as a marker of gastric differentiation.