O⁶-ethylguanine.

The Ames test is a protocol for identifying mutagenic chemical and physical agents. Mutagens generate changes in DNA. Many mutagenic agents modify the chemical structure of adenine, thymine, guanine, and cytosine, the bases in DNA, changing their base-pairing properties and causing mutations to accumulate during DNA synthesis.

Ethyl methanesulfonate (EMS), for example, is a very potent mutagen. The ethyl group of EMS reacts with guanine in DNA, forming the abnormal base O⁶-ethylguanine. During DNA replication, DNA polymerases that catalyze the process frequently place thymine, instead of cytosine, opposite O⁶-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.

During the 1960s the biologist Bruce Ames developed a test that...