Mucin-1 (MUC-1), cell surface associated or polymorphic epithelia mucin, is a 500-1000 kD proteoglycan expressed by activated T cells, mucosal epithelial cells, and aberrantly expressed on most breast cancers. In normal cells, CD227 is heavily glycosylated, whereas in cancerous cells, the glycosylation is incomplete and premature sialation is also observed. The protein is anchored to the apical surface of the epithelial cell and functions as a lubricant to keep the cell hydrated and to protect against pathogens. It can also function as a signaling molecule by forming a MUC-1/SOS1/GrB2 complex. MUC-1 can interact with cancer antigens such as Her2/neu.