Integron
The first part of an integron is a gene which codes for an enzyme which captures cassettes. The second is a site on the genome where the cassettes are inserted, and a promoter which drives expression of cassette-associated genes. "Integron" describes such structures both when no cassettes are present and when there are integrated cassettes. Cassettes can be inserted at the site, they can be cut out, and they can undergo horizontal gene transfer.