Glutathione peroxidase 3 (GPX3, GSHPX3) is a selenium-dependent extracellular enzyme, protecting cells and enzymes from oxidative damage by catalyzing the reduction of hydrogen peroxide, lipid peroxides and organic hydroperoxides. GPX3 is involved in inflammatory bowel disease, in asthma and in diabetes. The activity of GPX3 is significantly reduced in the plasma and tissue of cancer patients. Recently, GPX3 was suggested to serve as a molecular marker for the diagnosis of clear cell type ovarian adenocarcinoma.