To gain more knowledge of the mechanism of RNA recombination, several model experimental systems have been created. They provided us with some specific data describing homologous and/or non-homologous recombination in particular viruses, e.g. in poliovirus, (19) mouse hepatitis virus (20, 21) , brome mosaic virus (BMV) (4, 22) , turnip crinkle virus (23, 24) or tomato bushy stunt virus (25) . As a result, the involvement of viral replicase proteins in recombination has been demonstrated (26, 27) and a wide spectrum of RNA motifs supporting recombination have been identified (4, 23, (28) (29) (30) . In general, the collected data suggest that there exist two major types of RNA structural elements that induce recombination events: (i) universal ones mediating template switching by different viral replicases, e.g. regions *To whom correspondence should be addressed. Tel: +48 61 8528503; Fax: +48 61 8520532; Email: marekf@ibch.poznan.pl The online version of this article has been published under an open access model. Users are entitled to use, reproduce, disseminate, or display the open access version of this article for non-commercial purposes provided that: the original authorship is properly and fully attributed; the Journal and Oxford University Press are attributed as the original place of publication with the correct citation details given; if an article is subsequently reproduced or disseminated not in its entirety but only in part or as a derivative work this must be clearly indicated. For commercial re-use, please contact journals.permissions@oupjournals.org of local homology (28, 31) or complementarity (32) (33) (34) (35) and (ii) virus-specific ones, e.g. promoter-like structures (36, 37) . Unfortunately, up till now there has been no in vivo recombination system that could be used to test the recombination activity of any given RNA sequence and consequently to verify the above hypothesis and find some general laws governing the studied process.