Mucins, the major components of mucus, are high molecular weight glycoproteins that covers the surfaces of epithelial tissues. The MUC5AC gene, located on chromosome 11p15.5 was initially cloned from tracheobronchial and stomach cDNA libraries. MUC5AC is primarily expressed in gastric and tracheobronchial mucosa. Specifically, high levels are localized in foveolar cells within the gastric epithelium and goblet cells in the trachea. MUC5AC expression was also reported in the gall bladder and endocervical epithelium.

Expression of mucin antigens are often altered in carcinomas due to changes in glycosylation. Neoplastic transformation has been associated with a decrease in MUC5AC expression. A study from Reis and colleagues in gastric carcinomas demonstrated loss of MUC5AC expression in over 50% of tumor cells.