MUC1 and breast cancer: Exosomes carry a variety of tumor-specific proteins on their surface, such as MUC1, which is highly aberrantly expressed in breast cancer; human epidermal growth factor receptor-2, an important predictive and prognostic marker of breast cancer; EpCAM, which is highly expressed in almost all adenocarcinomas; and carcinoembryonic antigen, one of the independent prognostic indicators of breast cancer.