Class III elements (Figure 25.35) belong to a small group of bacteriophages, of which the best-known is phage Mu. This phage is known to insert its chromosome at random in the host chromosome by a transpositional mechanism and also to replicate its genome by a transpositional mechanism, similar to that of type II elements. One gene of this phage, A, encodes a transposase. Another gene, B, encodes a protein with DNA-dependent ATPase activity. Whereas class I and class II transposable elements synthesize transposase at such low levels that transposition occurs at frequencies of only 10^-7 to 10^-5 per generation, phage Mu integrates about 100 times per lytic infection. The B gene product is partially responsible for this far greater efficiency of transposition. Other genes encode structural and other proteins of the virus.