Ethyl Methane Sulfonate (EMS( 
 
EMS is a mutagen and a monofunctional alkylating agent of structure CH₃–CH₂–O–SO₂–CH₃, which reacts with nucleophilic sites on DNA by both S_N1 and S_N2 mechanisms. The SN1 process involves slow generation of a very reactive intermediate that alkylates DNA with only some discrimination for nitrogen atoms. Thus treatment of DNA or whole cells with EMS leads to many
DNA products. The most significant for mutagenesis is probably O⁶-ethylguanine, which base-pairs readily with thymine (1). Using bacteriophage systems, Loveless (2) showed that EMS is capable of producing high mutation frequencies, with little cell killing. The mutations produced were primarily G.C → A.T transitions, and to a lesser extent transitions containing A.T base-pairs at the mutated site, as well as frameshift mutations (3, 4). These latter events may relate to effects on DNA repair processes, while the initial events may be due to the base-pair miscoding potential of the O6-alkylguanine residues (1). EMS has often been considered the mutagen of choice for induced mutagenesis studies, being potent, easy to use, and having a well-documented mutational specificity.
References
1. P. F. Swann (1990) Mutat. Res. 233, 81–94. 
2. A. Loveless (1958) Nature 181, 1212–1213. 
3. E. Bautz and E. Freese (1960) Proc. Natl. Acad. Sci. USA 46, 1585–1594.
4. E. Freese (1961) Proc. Natl. Acad. Sci. USA 47, 540–545.