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.