Integrons are unusual DNA elements which include a gene encoding a site-specific DNA recombinase, a DNA integrase, and an adjacent site at which a wide variety of antibiotic resistance and other genes are found as inserts. One or more genes can be found in the insert region, but each gene is part of an independent gene cassette. The inserted genes are expressed from a promoter in the conserved sequences located 5' to the genes, and integrons are thus natural expression vectors. A model for gene insertion in which circular gene cassettes are inserted individually via a single site-specific recombination event has been proposed and verified experimentally. The gene cassettes include a gene coding region and, at the 3' end of the gene an imperfect inverted repeat, a 59-base element. The 59-base elements are a diverse family of elements which function as sites recognized by the DNA integrase. Site-specific insertion of individual genes thus represents a further mechanism which contributes to the evolution of the genomes of Gram-negative bacteria and their plasmids and transposons. Members of the most studied class of integrons, which include the sulI gene in the conserved sequences, are believed to be mobile DNA elements on the basis that they are found in many independent locations, and a discrete boundary is found at the outer end of the 5'-conserved segment. However, the length of the 3'-conserved segment is variable in the integrons examined to date, and it is likely that this variability has arisen as the result of insertion and deletion events. Though the true extent of the 3'-conserved segment remains to be determined, it seems likely that these integrons are mobile DNA elements. The second known class of integrons comprises members of the Tn7 transposon family.