Glutathione S-transferases (GSTs) are detoxification enzymes which modify and solubilize electrophilic toxins. These enzymes remove carcinogenic substances from the bloodstream and are involved in the breakdown of some therapeutic drugs. Thus they can play a role in cancer prevention and drug delivery. So far, researchers have failed to crystallize a mammalian theta class GST. We have succeeded in crystallizing the theta class GST 12-12 from rat liver. Due to the extreme radiation sensitivity of these unique crystals, a combination of synchrotron radiation and crystal cooling are likely to greatly improve the data that can be generated from them.