CA 125

Molecule Characteristics

Name: Carbohydrate Antigen 125
Gene: MUC16
Normal Function: Thought to create barrier against infective agents. Binds strongly to MSLN which is expressed in mesothelial cells


CA125 is the detectable product of the MUC16 gene (mucin 16, cell surface associated). Its normal function is thought to be protective by forming a barrier against infectious agents. Ovarian cancers frequently produce large quantities of this protein that can be detected in the blood. MUC16 is known to bind to MSLN (mesothelin), a protein that is expressed on the surface of mesothelial cells. The binding of MUC16 to MSLN is hypothesised to assist ovarian cancer in metastasising throughout the peritoneum.
CA125 is elevated in the following malignancies:

  • Ovarian cancer
  • Other gynaecological cancers (fallopian tube, endometrium)
  • Upper GI cancers (pancreas, liver, gastric, oesophagus)
  • Other common cancers (breast, lung)

Unfortunately CA125 is also elevated in a number of benign diseases:

  • Gynaecological conditions (pregnancy, menstruation, fibroids, cysts, PID)
  • Fluid collections (ascites, pleural effusion, pericardial effusion)
  • Cirrhosis

Use of CA125 testing

CA125 is not useful for screening due to the large number of common, benign conditions associated with elevated levels. It also lacks sensitivity for early stage ovarian cancer (50%). It does have a role as a diagnostic aid in a woman with an undiagnosed pelvic mass. The best use is for monitoring of disease progression following treatment for ovarian cancer.