CA125 is a high molecular weight glycoprotein belonging to the mucin family. It is a well-established tumor-associated antigen that is overexpressed in the majority of epithelial ovarian cancers and several other adenocarcinomas, while its expression in normal adult tissues is highly restricted. Its primary biological function involves providing a protective, lubricating barrier on epithelial cell surfaces. In oncology, CA125 serves as a critical diagnostic and monitoring biomarker for ovarian cancer, with serum levels correlating with tumor burden and response to therapy.