CA 125 is a protein that is encoded by the MUC16 gene in humans. MUC16 or Mucin 16 plays an important role in forming a protective mucus barrier. The full form of CA 125 is “cancer antigen 125” and it is a protein found in the ovarian cancer cells. CA 125 test is done to monitor certain cancers after the treatment as well as during the treatment. CA 125 test can also be used to look for early signs of ovarian cancer among the people who are in the high-risk category.