The genus Lactococcus contains four species of which L. lactis is the most thoroughly studied. Its genome is A+T-rich and consists of a circular chromosome of 2.0 to 2.7 Mbp, a wide range of plasmids, and frequently one or more prophages. Insertion sequence elements are commonly present in both the chromosome and the plasmids, while one conjugative transposon of 68 kbp has been described. Genetic maps of the chromosomes of a number of different L. lactis strains have been determined and found to differ quite significantly. For example, the maps of two L. lactis subsp. cremoris strains, MG1363 and FG2, have an inversion of approximately 40% of the chromosome when compared with the maps of two L. lactis subsp. lactis strains, DL11 and IL1403. Other differences indicative of smaller scale translocations and inversions are also found. Chromosomal rearrangements have also been induced in the laboratory by incubation of L. lactis cultures with infecting lytic phage and by treatment with mutagens. These results are indicative of a great deal of plasticity in the lactococcal genome.