The 5E11.3.1 antibody reacts with an unidentified marker expressed on the surface of most luminal cells of normal human mammary epithelium. Weak labeling (or no labeling) is observed within the basal cell compartment of the mammary epithelium. The 5E11.3.1 antibody gives variable labeling on malignant breast epithelium. Positive labeling is observed on the following cell lines: MCF7, CAMA, T47D, BT-10, BT-20, COLO 205, and WiDr. Labeling is negative on human bone marrow cells, blood cells, lymphoid and mesenchymal tissues, mammary and marrow fibroblast cultures, and human embryonic stem cell line H9.