Crop selectivity is important in allowing weeds to be controlled by herbicides without damage to the crop. Often tolerant crops have more rapid and complete detoxification of a herbicide than occurs in sensitive crop species and weeds. A number of enzymatic systems are able to detoxify herbicides in plants. These include aryl acylamidases that cleave propanil, glutathione reductases that add the tripeptide glutathione to herbicides and cytochrome P450 monooxygenases that hydroxylate herbicides. O-Glycosyltransferases may further detoxify hydroxylated herbicides. Glycosylation and glutathione conjugation of herbicides are signals for extrusion of compounds from the cytoplasm, either into the vacuole or into the apoplast. Occasionally intensive use of herbicides will select weed populations with resistance to herbicides as a result of a greater capacity to detoxify herbicides. The types of enzymes responsible for detoxification of herbicides by weeds are the same as those responsible for herbicide selectivity in crops, but the regulation of these enzymes may be different.