resistance to the antibiotic tetracycline. This activity of tetR forms the basis of the TetOFF assembly, a widely-used construct in synthetic gene studies. The TetA protein is an efflux pump. It has served as a model system for such proteins and has accumulated hundreds of publications indexed in PubMed. The functions of the other three genes, jemA, jemB and jemC, are unknown but they may implicated in heavy metal resistance or oxidative stress. The Tn10/IS10 transposase is closely related to another composite transposon, Tn5/IS50, which harbors a gene for kanamycin resistance in the unique (i.e. non-repeated) central region of the transposon. The Tn10 transposon is often used in genetics to transfer and select-for genes of interest from one organism into the chromosome of another. The mechanism of Tn10 transposition has served as a model system and the archetype for the cut-and-paste mechanisms. However, the transposase is difficult to work with in vitro and the Tn5 transposase was the first to be crystallized. Tn10 was one of the great work-horses of bacterial genetics for many years during which it served as a useful tool. A commercial kit for Tn5 transposition is commercially available and is extensively used in post-genomic technologies. Two comprehensive reviews of Tn10 biology are available as chapters in the book Mobile DNA and Mobile DNA II. References DNA mobile genetic elements