Mucin 1 (MUC1, epithelial membrane antigen) is a membrane bound mucin glycoprotein expressed by a variety of glandular and ductal epithelial cells, including those of the breast, lung, gastrointestinal tract kidney, and pancreas, as well as activated T cells, plasma cells, monocytes, some B cells, follicular dendritic cells, and perineurial cells. In normal epithelia, it is expressed on the apical surface and is involved in forming a protective barrier as well as cell signaling. MUC1 is useful for identifying most adenocarcinomas, anaplastic large-cell lymphomas, epithelioid sarcomas, meningiomas, some mesotheliomas, myelomas, Paget disease, plasmacytomas, squamous-cell tumors, and metastatic carcinomas. References: Pathology 2003, 35(5):422; Modern Pathology 2005, 18:1295