Mucin 4 (MUC4) is a highly glycosylated type I transmembrane glycoprotein. Normally acts as barrier to apical surface of epithelial cells, playing a protective and regulatory role. MUC4 expression is erroneous in many carcinomas and sarcomas, including sclerosing epithelioid fibrosarcoma and pancreatic adenocarcinoma. Immunohistochemical staining for MUC4 has been discussed as a potential biomarker for detection of these cancers. MUC4 can serve as a ligand for the oncogenic receptor ErbB2 and a modulator of its phosphorylation and signaling. MUC4 is frequently aberrantly expressed in epithelial tumors and can promote tumor growth and metastasis.