General structure of the integron/gene cassette system. This representation is consistent with the single-strand cleavage model proposed by MacDonald et al., 2006 . The secondary structure illustrated for part of the free gene cassette (center) is achieved through base-pairing of the bottom DNA strand within the attC-encoded imperfect repeats (L', L" and R', R"). The IntI binding sites in attI known to be required (L, R) and accessory (DR1, DR2) for attC × attI recombination are also illustrated [16, 17]. The diagram is based on functional studies of class 1, class 3 and Vibrio integrons and might not apply to all integron types.