The CA125, also known as the MUC16, is a mucin protein that may be found in type I transmembrane or secreted forms that are used monitor the progress of epithelial ovarian cancer therapy. However，due to lack of sensitivity for stage I disease and lack of specificity, it is of little value in the detection of early ovarian cancer. Due to its similarly elevated levels in some nonmalignant conditions, it is not specific enough to be used for population screening. It is thought to provide a protective, lubricating barrier against particles and infectious agents at mucosal surfaces. The CA 125 molecule is almost certainly a glycoprotein with a predominance of O-linkages. It is heterogeneous with regard to both size and charge, most likely due to continuous deglycosylation of side chains during its life-span in bodily fluids. It exists as a very large complex (perhaps as much as 4 million daltons) under natural conditions.