Insertion sequence IS10, the active transposable element of transposon Tn10, is one of the best characterized mobile genetic elements. Studies on IS10 have revealed many aspects of gene expression that are of general importance to molecular biologists, including antisense RNA control, the focus of much of the research efforts described here. The domains of the antisense RNA that determine its unusual stability and its efficient pairing to the transposase mRNA are now known, a detailed model for the pairing reaction, which differs from other well-defined cases, has been proposed, and it is now known that this antisense RNA works by inhibiting ribosome binding and facilitating mRNA decay. In the midst of these studies, two other important post-transcriptional aspects of IS10 gene expression have been revealed. Both mechanisms involve intramolecular structures in the transposase mRNA. One is a short-range RNA/RNA interaction that prevents translation of RNAs initiated outside the IS10 element. The second is a long-range RNA/RNA interaction that restricts antisense binding and, probably, ribosome binding, to nascent mRNAs. In general, translational control of gene expression involves sequences and structures in RNA, as well as regulatory functions (RNA, protein or otherwise) that bind to and alter these sequences and structures or alter the integrity of the RNA itself. Many of these features remain ill-defined. Indeed, there remain a number of important questions concerning IS10 translational control. In the work proposed here, a genetic and biochemical assault will be mounted on issues of importance to antisense pairing and the short and long-range interactions in the transposase mRNA. Focus will be on the general question of how RNA sequences, present on the same or on different molecules, interact to form biologically relevant structures. Emphasis will be on the pathway, speed, stability and modulation of structure formation.