Mucin 4 (MUC4) is a mucin protein that in humans is encoded by the MUC4 gene. Like other mucins, MUC4 is a high-molecular weight glycoprotein.  MUC4 belongs to the human mucin family that is membrane-anchored and can range in molecular weight from 550 to 930 kDa for the actual protein. MUC4 antibody labels normal epithelial cells in the trachea, GI tract  and prostate, but not in the pancreas.

MUC-4 has been found to play various roles in the progression of cancer, particularly due to its signaling and anti-adhesive properties which contribute to tumor development and metastasis. It is also found to play roles in other diseases such as endometriosis and inflammatory bowel disease. An abnormal expression of MUC4 has been reported in various carcinomas of the colon, pancreas, breast, and ovaries. Increased expression of MUC4 has been observed in pancreatic carcinoma and cervical squamous carcinoma. MUC4 is helpful in differentiating lung adenocarcinoma (positive) from malignant mesothelioma (negative). Additionally, MUC4 is useful in the identification of low-grade fibromyxoid sarcoma (LGFMS), and sclerosing epithelioid fibrosarcoma. MUC4 expression is also detected in the glandular component of biphasic synovial sarcomas.