Mucin-5AC is a protein that in humans is encoded by the MUC5AC gene. This gene has been linked to mucus hypersecretion in the pulmonary tracts and associated to chronic obstructive pulmonary disease (COPD). Gel-forming glycoprotein of gastric and respiratoy tract epithelia that protects the mucosa from infection and chemical damage by binding to inhaled microrganisms and particles that are subsequently removed by the mucocilary system. Highly expressed in surface mucosal cells of respiratory tract and stomach epithelia. Overexpressed in a number of carcinomas. Also expressed in Barrett's esophagus epithelium and in the proximal duodenum.