Ethyl methanesulfonate (EMS) is a mutagenic, teratogenic, and possibly carcinogenic organic compound with formula Template:Carbon3Template:Hydrogen8Template:Oxygen3Template:Sulfur. It produces random mutations in genetic material by nucleotide substitution; specifically by guanine alkylation. This typically produces only point mutations. It can induce mutations at a rate of 5x10-4 to 5x10-2 per gene without substantial killing. The ethyl group of EMS reacts with guanine in DNA, forming the abnormal base O-6-ethylguanine. During DNA replication, DNA polymerases that catalyze the process frequently place thymine, instead of cytosine, opposite O-6-ethylguanine. Following subsequent rounds of replication, the original G:C base pair can become an A:T pair. This changes the genetic information, is often harmful to cells, and can result in disease. Many mutagens cause a wide variety of cancers in humans.
EMS is often used in genetics as a mutagen. Mutations induced by EMS can then be studied in genetic screens or other assays.
In the 1982 movie Blade Runner, replicant Roy Batty asks Dr. Eldon Tyrell about the possibility of extending his preprogrammed lifespan by changing his genetic code with "EMS recombination". He is informed that ethyl methanesulfonate is a potent mutagen and that a test subject was "dead before he left the table."