Mechanism of transposition of bacteriophage Mu: polarity of the strand transfer reaction at the initiation of transposition. Cell 39:395–401

Nature abounds in gene rearrangement reactions. Some of the rearrangement reactions can be consid-ered, from a mechanistic point of view, as mistakes or accidents during general recombination reactions, DNA replication, or DNA damage repair p ocesses. These reactions do not involve proteins or factors whose primary function is to promote the rearrange-ment reactions, and many of such reactions do not oc-cur reproducibly. In contrast, a movable genetic ete-ment is a defined segment of DNA that can be moved from one context o another and is associated with its own protein factor or factors whose function is, at least, to initiate movement of this specific element. The mode of action of the specific protein factor might be as simple as sequence-specific cleavage of DNA strand