MUC1 and breast cancer: MUC1 is a tumor-associated protein expressed by most solid tumors, including more than 90% of breast and pancreas tumors as well as multiple myelomas and lymphomas; even early stage triple negative breast tumors show almost uniform expression of this antigen [9], although others have shown that MUC1 expression is heterogeneous and reduced in clinically progressive triple negative breast cancers compared with luminal breast cancers [10].