In the late 1960s microbial geneticists started to pay attention to a class of spontaneous mutations which often exerted polar e¡ects and which turned out to be caused by the insertion of mobile genetic elements [1]. Subsequent studies on transposition of mobile elements made it clear that by far not all microbial recombination activities concerned homologous recombination occurring between identical DNA sequences. As a matter of fact, site-speci¢c recombination had already been studied at that time, particularly the integration of proviral genomes into the bacterial host chromosome [2]. It was then revealed that sitespeci¢c recombination can also occur, although at lower frequencies, at DNA sequences deviating from the normal recombination site. Both DNA rearrangement activities mentioned here, transposition and site-speci¢c recombination, can also give rise to deletion formation as well as DNA inversion. In addition, both of these activities can bring about an association of segments of chromosomal DNA with viral genomes and plasmids which thus act as natural gene vectors. Studies on specialized transduction, FP conjugative plasmids and antibiotic resistance plasmids were very instructive in this regard