Human glutathione S-transferases (GSTs) are a large multigene family of isoenzymes, which catalyse the conjugation of glutathione to electrophilic substrates. These enzymes are involved in the detoxification of both endogenous and exogenous electrophiles, which can react with cellular components such as DNA. The modification of DNA by reactive compounds can initiate carcinogenesis and the GSTs are believed to play a role in cancer prevention by inactivating carcinogens. There are four major structural classes of mammalian cytosolic GSTs (alpha, mu, pi and theta).