Cancer antigen 125 or CA-125 is a protein. Although it occurs in everyone, in individuals with certain types of cancer, it's levels are elevated and can used in the diagnosis, prognosis and ongoing assessment of certain types of tumor. Most typically, it indicates ovarian cancer. However, elevation of the protein is not very specific and can also indicate conditions that are benign. As such, it's use in early detection of ovarian cancer is questionable at best. It can also be triggered by cancers of other tissues that have similar structures, such as the fallopian tubes and even the intestines.

The relationship of CA-125 to cancer is genetic as the gene that codes for CA-125 also plays a role in hiding tumors from the immune system and even protecting them from chemotherapy.

CA-125 at Wikipedia