The 16A antibody reacts with human MUC1, a large (> 250 kDa) heavily glycosylated type 1 transmembrane protein expressed on the surface of most glandular and ductal epithelial cells and a variety of hematopoietic cells. A characteristic feature of the MUC1 glycoprotein is a core domain composed of a variable number of tandem repeats and multiple oligosaccharide side chains. Because the extracellular portion of MUC1 can extend beyond most cell surface proteins, it is thought to play a role in cell-cell and cell-substrate adhesion. The protein is highly expressed by a majority of human adenocarcinomas and is associated with a poor prognosis. In the mammary gland, MUC1 is localized on the apical plasma membrane of luminal epithelial cells. The clone 16A antibody has a higher affinity for the glycosylated form of MUC1.