Common substrates that are used as terminal electron acceptors in organohalide respiration are organochloride pesticides, aryl halides and alkyl solvents. Many of these are persistent and toxic pollutants that can only be degraded anaerobically by organohalide respiration, either partially or completely. Trichloroethylene (TCE) and tetrachloroethylene (PCE) are two examples of such pollutants, and their degradation has been a focus of research. PCE is an alkyl solvent that was previously used in dry cleaning, degreasing machinery and other applications. It remains a common contaminant of groundwater. Bacteria that are capable of completely degrading PCE to ethene, a nontoxic chemical, have been isolated. They have been found to belong to the genus Dehalococcoides and to use H as their electron donor. The process of organohalide respiration has been applied to in situ bioremediation of PCE and TCE in the past. For example, enhanced reductive dechlorination has been used to treat contaminated groundwater by introducing electron donors and dehalorespiring bacteria into the contaminated site, to create conditions that stimulate bacterial growth and organohalide respiration.  In enhanced reductive dechlorination, the pollutants act as the electron acceptors and are completely reduced to ultimately produce ethene in a series of reactions.