The high level of genetic instability in Streptomyces ambofaciens is related to large scale DNA rearrangements (deletions and DNA amplifications) which occur within a 2 Mb chromosomal region. The genome of several Streptomyces species is linear and the unstable region is present at the chromosomal extremities. This has raised the questions of the role of the unstable region (which is dispensable under laboratory conditions), the functions of the genes present in this area, and the relationships between instability and chromosomal linearity. The unstable region of Streptomyces and the replication termini of several other microorganisms, including Escherichia coli, share numerous common traits. This suggests that the unstable region of Streptomyces includes the replication terminus, and that chromosomal instability is related to the termination process.