The glutathione S-transferase (GST) family of enzymes comprises a long list of cytosolic, mitochondrial, and microsomal proteins that are capable of multiple reactions with a multitude of substrates, both endogenous and xenobiotic. Mammalian cytosolic GSTs are dimeric both subunits being from the same class of GSTs. The monomers are in the range of 22–29 kDa. Glutathione S-transferase is used to create the “GST gene fusion system” in genetic engineering. Here, GST is used to purify and detect proteins of interest.