Twenty-nine IMP-type β-lactamases (IMPs) have been identified in at least 26 species of clinically important Gram-negative bacilli from more than 24 countries/regions. Most of bla(IMP) genes are harbored by class 1 integrons that are usually embedded in transposons and/or plasmids, footnoting their horizontal transfer and worldwide distribution. bla(IMP) genes usually co-exist with other resistance genes, such as aacA, catB, and bla(OXA), resulting in multi-drug resistance. Compared to other gene cassettes, 76.3% of the bla(IMP) gene cassettes are located adjacent to Pc promoter of the class 1 integrons, indicating that the bla(IMP) genes are readily expressed in most of bacterial hosts.