Insertions of Tn10 are characterized by the presence of a direct repetition of a 9-bp target gene sequence on either side the insertion. The occurrence of these repetitions undoubtedly reflects an important feature of the way in which DNA molecules are broken and joined during translocation. Our experiments further suggest, however, that these 9-bp sequences are probably not responsible for Tn10-insertion specificity and that their presence is not required for normal Tn10 translocation elsewhere. We therefore suggest that the genetic information which controls the quality and quantity of Tn10 translocation actually resides somewhere other than these 9-bp sequences. We presume that much of this information lies within the ends of Tn10 itself and that signals on the target DNA which guide Tn10 to preferred positions must occur near, but not actually at, the eventual physical site of the integration event. Consideration of Tn10-promoted deletions and inversions reemphasizes the role of these ends in Tn10-promoted recombination events. Since Tn10-promoted events almost always consist in joining the physical end of one of the putative IS sequences of Tn10 to some other DNA segment, one comes again to the conclusion that these ends must contain important genetic information governing recombination events.