ERBB2 and breast cancer: Several such antigens have been detected in the serum of BC patients including carcinoembryonic antigen (CEA), CA15-3 (41), CA27-29, CA-125, Trop2 (42), tissue polypeptide specific antigen (TPS) (43), the circulating extracellular domain of HER2, and riboflavin carrier protein (RCP) (44).