MUC16 (CA125) is a serum marker that is used routinely in gynecologic practice to monitor patients with ovarian cancer. It is a mullerian duct differentiation antigen that is overexpressed 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 MUC16 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.