Glutathione (GSH) transferases catalyze the conjugation of the tripeptide GSH with alkyl halides and related compounds. If a second leaving group is present, the substrate is at least a potential bis-electrophile and the initial conjugate may be susceptible to further attack by the sulfur atom. This process can yield potent electrophiles that modify DNA and are genotoxic. Much of the chemistry is understood in the context of the halide order and size of rings generated in reactive sulfonium ions. Similar chemistry has been demonstrated with the active site cysteine residue in the DNA repair protein O(6)-alkylguanine DNA-alkyltransferase.