Mucin 1, cell surface associated (MUC1) or polymorphic epithelial mucin (PEM) is a mucin, the protein part of which is encoded by the MUC1 gene in humans. Mucin proteins penetrate the membranes of epithelial cells, on the inner surface of the intestine and other organs. Mucin protects the body from infection by binding to pathogens. Overexpression of the mucin protein is often associated with colon and many other cancers.