Cells produce many specific endogenous antioxidant systems to deal with conditions of oxidative stress. The most common and often measured include, glutathione (GSH, GPX and GR), superoxide dismutase (SOD1, SOD2, SOD3 & SOD4) and catalase. Other systems include thioredoxin (TRX, TRR) and peroxiredoxin (PRX1, PRX3 & others).