Antigen carbohydrate CA125 is a tumor marker used in the monitoring of epithelial ovarian cancers. It is a transmembranaire glycoprotein with a high molecular weight. It is a mucin with a carbohydrate content estimated to be 24-28%. CA125 is encoded by the gene MUC16 which is rich of tandem repeats. Different hypothesis were suggested concerning the mechanism of secretion of CA125 that could be soluble or attached to the cellular surface. This last form of CA125 has many molecular interactions such as with mesothelin and galectin. These interactions play a key role in the stimulation and the dissemination of ovarian cancer via the tumor marker CA125.