The MUC 1 antigen is a membrane-anchored, mucin-type glycoprotein present in malignant and normal epithelial cells of certain organs (ie, breast, lung, ovary, pancreas, and colon). The apoprotein of the MUC 1 mucin contains a transmembrane domain, a cytoplasmic domain, and an extracellular carbohydrate-rich domain. The extracellular domain is characterized by polymorphism with respect to the number of a 20 amino acid tandem repeat (VNTR polymorphism). The CA15-3 EIA is based on two mouse monoclonal antibodies, Ma695 as catcher antibody recognizing a sialylated carbohydrate epitope expressed on the MUC 1 antigen and Ma552 as tracer antibody targeting the PDTRPAPG region of the protein core.The MUC 1 breast cancer mucin (CA15-3 antigen) is secreted from tumor cells and is a well-established serological marker for monitoring the clinical course of breast cancer patients.