Well studied examples are the viruses that infect bacteria, known as bacteriophages (or simply phages). Bacteriophages, known as phages, are a form of viruses. This process, known as multiplicity reactivation, has been studied in several bacteriophages, including phage T4. A set of tests with bacteriophages reveals that one phage can kill Andromeda. It often involves temperate bacteriophages and plasmids. Transposon sequencing begins by transducing bacterial populations with transposable elements using bacteriophages. Virulent bacteriophages multiply in their bacterial host immediately after entry. These sequences are derived from DNA fragments of bacteriophages that had previously infected the prokaryote. Dam methylase is unique to prokaryotes and bacteriophages. Actinomycetales bacteria can be infected by bacteriophages, which are called actinophages. Class IA reductases are distributed in eukaryotes, eubacteria, bacteriophages, and viruses. Twort discovered the action of bacteriophages on staphylococci bacteria. D'Herelle travelled widely to promote the use of bacteriophages in the treatment of bacterial infections. Operons are also found in viruses such as bacteriophages. Class III reductases are distributed in archaebacteria, eubacteria, and bacteriophages.