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Enzymes of the glutathione S-transferase (GST) family are composed of many cytosolic, mitochondrial and microsomal (now designated as MAPEG) proteins. GSTs are present in eukaryotes and in prokaryotes, where they catalyze a variety of reactions and accept endogenous and xenobiotic substrates. GSTs can constitute up to 10% of cytosolic protein in some mammalian organs. GSTs catalyse the conjugation of reduced glutathione, via a sulfhydryl group, to electrophilic centers on a wide variety of substrates. This activity detoxifies endogenous compounds such as peroxidised lipids as well as breakdown of xenobiotics. GSTs may also bind toxins and function as transport proteins, and, therefore, an early term for GSTs was “ligandin”. The mammalian GST super-family consists of cytosolic dimeric isoenzymes of 45–55 kDa size that have been assigned to at least six classes: Alpha, Mu, Pi, Theta, Zeta and Omega. GST tags are commonly used as epitope tags in recombinant protein expression and purification.