Previous immunocytochemical and immunoprecipitation studies have detected the MUC1-C subunit (which comprises the cytoplasmic domain, the transmembrane domain, and a short extracellular domain that does not include the VNTR region) in the nucleus of breast cancer cell lines and breast cancer tissue but failed to detect MUC1 extracellular domain (i.e., the N-terminal subunit) in that location. Here, MUC1 is linked to breast cancer.