Genetical tests and DNA sequence analysis revealed that the mechanism of formation of IS1-induced type I and type II deletions differs. IS1-mediated type II deletions occur at the termini of the integrated element and do not remove the element. This process is independent of the cellular recA system and does not involve DNA sequence homology. Conversely, the formation of IS1-induced type I deletions differs substantially. They require recA gene product, small DNA sequence duplications and a topological arrangement of the DNA molecule to allow alignment of duplications.