MUC5AC encodes a gel-forming glycoprotein that serves as a major secreted mucin in respiratory and gastric epithelia 1. This large, heavily glycosylated protein is organized with von Willebrand factor-like D domains at its N-terminus, followed by a central repetitive region containing cysteine-rich domains and tandem repeats 2. MUC5AC functions to protect mucosal surfaces by binding to inhaled microorganisms and particles that are subsequently cleared by the mucociliary system 3. In airway epithelium, MUC5AC works cooperatively with MUC5B to form a functional hydrogel; while MUC5B mobilizes mucus transport, MUC5AC spatially aligns mucociliary clearance, and deficiency of either mucin impairs coordinated mucus transport 3. Glucocorticoids transcriptionally repress MUC5AC expression through glucocorticoid receptors and histone deacetylase-2 recruitment to specific promoter elements, providing a mechanism for therapeutic reduction of excessive mucin production in obstructive airway diseases 4. IL-13 induces MUC5AC expression via STAT6-dependent and independent pathways 5. Pathological upregulation of MUC5AC occurs in chr11 respiratory diseases and contributes to airway occlusion; environmental toxins like fipronil can elevate expression through MAPK and NF-κB signaling 6. Additionally, MUC5AC interacts with various drugs at the mucosal surface, potentially affecting drug absorption 7.