The 214D4 antibody reacts with MUC1 (also known as CD227), a large cell surface glycoprotein expressed by most glandular and ductal epithelial cells as well as a variety of hematopoietic cells. A characteristic feature of the MUC1 (CD227) glycoprotein is a core protein 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 the 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 214D4 antibody clone is particularly useful since it enriches for luminal-restricted but not bipotent epithelial progenitor cells from normal human mammary tissue.