Cells or organisms may be exposed to mutagens such as UV radiation or mutagenic chemicals, and mutants with desired characteristics are then selected. Hermann Muller discovered that x-rays can cause genetic mutations in fruit flies (published in 1927),[2] and went on to use the Drosophila mutants created for his studies on genetics.[3] For Escherichia coli, mutants may be selected first by exposure to UV radiation, then plated onto agar medium. The colonies formed are then replica-plated, one in rich medium, another in minimal medium, and mutants that have specific nutritional requirements can then be identified by their inability to grow in minimal medium.