CA-125, or MUC16, is a serum marker that is used routinely in gynecologic practice to monitor patients with ovarian cancer. CA-125 is normally found in epithelial cells of Fallopian tube, endometrium and endocervix, pancreas, colon, gall bladder, stomach, kidney, apocrine sweat gland, mammary gland, and mesothelial cell lining of pleura, pericardium and the peritoneum. It is a mullerian duct differentiation antigen that is over expressed in epithelial ovarian cancer cells and secreted into the blood, although its expression is not entirely confined to ovarian cancer. Most biochemical studies have concluded that CA-125 is a high molecular mass glycoprotein, although estimates of its size range from 200 to 2000 kDa with smaller 'subunits' being described by some investigators. Staining with CA-125 reacts positively with ovarian malignancies, cervical carcinoma, endometrial and bladder adenocarcinoma, seminal vesicle carcinoma, and anaplastic lymphoma.