10072 Background: MUC1, a glycoprotein highly expressed in epithelial malignancies including breast, prostate, and ovarian, and on the malignant cells of multiple myeloma has generated considerable interest as a tumor marker and target for tumor killing. The most intensively studied MUC1 protein is a type I transmembrane protein (MUC1/TM) which is proteolytically cleaved soon after synthesis into α and β subunits which bind in a strong non-covalent interaction. Almost all antibodies generated to date against MUC1 recognize epitopes within the highly immunogenic tandem-repeat-array. A major shortcoming in use of such antibodies is the fact that the tandem-repeat-array-containing part of MUC1 is shed from the cell surface into the circulation. Soluble, shed MUC1 sequesters circulating anti-tandem-repeat-array antibodies, limiting their ability to reach targeted MUC1-expressing cells. Antibodies to MUC1 epitopes tethered to the cell surface would likely be more effective therapeutic agents. Despite efforts in recent years, such antibodies have remained elusive; generation of anti-cell antibodies requires characterization of cell-bound epitopes. The junction of the MUC1 α-subunit binding the membrane-tethered β-subunit provides such an epitope. Methods: By use of a novel protocol, entailing immunization with MUC1/TM cDNA and boosting with MUC1/X protein, a MUC1 isoform lacking the tandem-repeat-array, we generated monoclonal antibodies designated DMC209 which recognize the MUC1 α/β junction. Results: DMC209 is exquisitely unique for the target site; all amino acid mutations which abrogate MUC1 cleavage also abrogate DMC209 binding. Additionally, DMC209 binds the MUC1 α/β junction on cell-tethered tandem-repeat-array-containing MUC1 (MUC1/TM) on breast and ovarian cancer, and on myeloma cells. Conclusion: DMC209 is likely to have clinical application by targeting MUC1-expressing cells directly, and as an immunotoxin conjugate. Moreover, the novel high-titer immunization procedure used in generating DMC209 can be used to generate anti-MUC1 α/β junction antibodies acting as ligand and, analogously to herceptin, may have direct cytotoxic activity. No significant financial relationships to disclose.