Nidovirus discontinuous minus strand extension resembles similarity-assisted, copy-choice RNA recombination Due to their discontinuous sg RNA synthesis, nidoviruses occupy a special`niche' in the +RNA virus world. Their mode of sg RNA production is clearly different from that of other +RNA viruses and resembles another welldocumented +RNA virus feature: RNA recombination (for recent reviews see Nagy and Simon, 1997; Aaziz and Tepfer, 1999; Worobey and Holmes, 1999) . Most of the experimental evidence supports an RdRp template switch (Kirkegaard and Baltimore, 1986) as the main mechanism of RNA recombination. Mechanistically, such a template switch involves the transfer of a nascent strand from one RNA template (donor) to the other (acceptor). Also, nidovirus discontinuous sg RNA synthesis involves transfer of a nascent RNA strand, the sg RNA, but now from one site to another in the same template.