Tn10 is a composite transposon. It comprises a pair of IS10 insertion sequences located in opposite orientation flanking ~6.7 kb of unique sequences; these unique sequences encode a tetracycline resistance determinant and other determinants whose functions remain to be identified (Fig. 1A; Kleckner 1989). One of Tn10’s two IS10 elements, IS10-Right, is structurally and functionally intact and is considered to be the “wild type” IS10. IS10 encodes a single transposase protein which mediates transposition by interacting with specific sequences at two oppositely oriented IS10 (or Tn10) termini. The termini of IS10 are subtly different and are referred to as the “outside” and “inside” end, respectively, by virtue of their position in Tn10. IS10-Left is structurally intact but encodes a substantially defective transposase.