Three types of bacterial transposons. (a) Insertion sequences (IS elements) are the simplest transposons, consisting of a transposase-encoding gene and a transposase-binding site within the inverted repeat (IR) sequence at either end. (b) Composite transposons usually consist of two IS elements flanking some additional genes. The transposon Tn5, with two IS50 elements flanking three genes specifying antibiotic resistance, is an example. (c) Complex transposons contain genes, in addition to the transposase gene, that are required for general maintenance. An example is bacteriophage Mu, which can function as either a very efficient transposon or a bacteriophage; attL and attR are the transposase-binding sites.