MUC5AC is a gel-forming glycoprotein that serves to protect the mucosa in the gastric and respiratory tract epithelia from infection and damage. It can bind to inhaled particles and infectious agents, which are then removed by the mucocilary system. MUC5AC also functions in epithelial cell proliferation and differentiation in adult airway mucosa and can be used as a marker for hypersecretory disorders. Expression of MUC5AC (along with MUC5AB) is altered in asthma and may contribute to its pathogenesis, and thus it may be a useful biomarker and potentially also a therapeutic target for the disease. MUC5AC has also been observed in various tumors. It is expressed in primary mucinous ovarian tumors but not in colonic carcinoma metastatic to the ovary, which makes it a useful marker to distinguish the two types of cancer. In immunohistochemistry of normal tissue, MUC5AC has secretory positivity in mucus-producing cells in the lung, gastrointestinal tract, gallbladder and cervix. References: J Clin Med. 2017 Dec; 6(12): 112, PMID: 29186064; Annu Rev Physiol. 2008;70:405-29, PMID: 17961085; Am J Respir Cell Mol Biol. 2007 Sep;37(3):273-90, PMID: 17463395; Hum Pathol. 2000 Jun;31(6):672-7, PMID: 10872659