Ruth Hall has made an exceptional contribution to Microbiology and Genetics by discovering and characterising a novel system for gene mobilisation that is critical for the dissemination of antibiotic resistance genes and is also a major generator of diversity in bacteria. The known integron types each consist of a backbone encoding a recombinase and a site where genes are integrated. The genes are in small mobile elements called gene cassettes that usually include only the gene and a recombination site. Over 100 resistance genes are in cassettes, but the total of all cassette types is known to be enormous and the genes to be extremely diverse.