Based on the data currently available, we refer to the discontinuous minus strand extension model as our working model for nidovirus sg RNA synthesis. If one applies the`recombination terms' to this model (Chang et al., 1996; Brian and Spaan, 1997; van Marle et al., 1999a) , the donor strand would be the body part of the genomic RNA template, the acceptor strand would be the leader part of the genomic RNA template and the nascent strand would be the discontinuously synthesized minus strand. Nagy and Simon (1997) have de®ned three main classes of RNA recombination: similarity-essential, similarity-non-essential and similarity-assisted recombination. The latter is de®ned as a mechanism in which strand transfer is determined by both sequence similarity between the parental RNAs and additional RNA determinants, present in only one of the parental RNAs.