MUC1 is a transmembrane protein normally expressed in glandular or luminal epithelial cells and hematopoietic cells and absent in skin epithelium and mesenchymal cells. Apical surface of almost all glandular and ductal epithelial cells, including respiratory, gastrointestinal, urogenital, hepatobiliary tracts, sebaceous and salivary glands. MUC1 is expressed in the central cells of pancreatic acini, intercalary duct epithelium, and interlobular duct epithelium, but not in the acinar epithelium, main pancreatic duct epithelium, and pancreatic islet epithelium. Overexpression is common in malignancies and leads to tumorigenesis. Increased expression is associated with poor prognosis of most carcinomas, including those involving breast, cervical, pancreatic, colorectal.