This MAb reacts with MUC1, a large transmembrane glycoprotein expressed on the ductal surface of normal glandular epithelia. It is used as tracer agent in CA15.3 assays. The extracellular domain of MUC1 largely consists of a highly conserved, O-glycosylated 20 amino acids tandem repeat which can occur 30-100 times per molecule depending on the length of the allele involved. In the vast majority of human carcinomas this protein is up-regulated and poorly glycosylated and appears on the cell surface in a non-polarized fashion. The dominant epitope of this MAb involves both amino acids as well as sugar moieties. Its epitope is destroyed by desialylation i.e. treatment with Neuraminidase.