ERBB2 and breast cancer: For example, in breast cancer, different serum markers are carcinoembryonic antigen (CEA), the soluble form of the MUC-1 protein (CA15-3), circulating cytokeratins, such as tissue polypeptide antigen (TPA), tissue polypeptide-specific antigen (TPS) and cytokeratin 19 fragment (CYFRA 21-1), and the proteolytically cleaved ectodomain of the human epidermal growth factor receptor 2 (s-HER2).