Precise excisions of transposon insertions occur as rare events by a mechanism(s) unrelated to transpostion or homologous recombination. We have identified and characterized mutations in ssb (encoding the single-strand DNA binding protein SSB), topA (encoding topoisomerase I) and a new gene called uup that increase the frequency of transposon precise excisions (1,2). The uup gene is the second gene of a bicistronic operon ycbY-uup, and the function encoded by ycbY has been shown not to be involved in transposon excisions (2). Mutations in uup also increase the frequency of another mutational event, namely deletion of one copy of a tandem chromosomal repeat (TRD). Future studies are aimed at understanding the physiological functions of the uup and ycbY genes, and at elucidating the mechanisms of precise excisions and TRD.