The occurrence of mutations can be increased by treatment with certain compounds. These are called mutagens, and the changes they cause are referred to as induced mutations. Most mutagens act directly by virtue of an ability either to act on a particular base of DNA or to become incorporated into the nucleic acid. The effectiveness of a mutagen is judged by the degree to which it increases the rate of mutation above background. By using mutagens, it becomes possible to induce many changes in any gene.