: The major constituents of mucus, the viscous secretion that covers epithelial surfaces such as those in the trachea, colon, and cervix, are highly glycosylated proteins called mucins. These glycoproteins play essential roles in protecting the epithelial cells and have been implicated in epithelial renewal and differentiation. This gene encodes an integral membrane glycoprotein found on the cell surface, although secreted isoforms may exist. MUC-4 transcripts have been detected in normal respiratory epithelium and lung. MUC-4 is a specific (100%) and sensitive (90%) marker of lung adenocarcinomas and is negative for mesotheliomas. Reportedly, MUC-4 expression in invasive ductal carcinoma of the pancreas is an independent factor for poor prognosis. MUC-4 is also explicitly expressed in low-grade fibromyxoid sarcoma and sclerosing fibrosarcoma.