The RNA±RNA interaction between the leader and body TRSs is not the only factor that regulates EAV sg RNA synthesis There are numerous examples of regulatory RNA±RNA interactions in both eukaryotic and prokaryotic cells, as well as in RNA viruses. Essential processes such as translation, replication and encapsidation of RNA virus genomes frequently depend on RNA±RNA interactions and higher order RNA structures. Regulation of sg RNA synthesis of +RNA viruses by RNA±RNA interactions is also not without precedent. In tomato bushy stunt virus, an RNA element located 1000 nucleotide upstream of the sg RNA2 promoter base-pairs with the promoter and is necessary for sg RNA production (Zhang et al., 1999) . Similarly, base pairing interactions between complementary sequences in the 5¢ end of the potato virus X genomic RNA and sequences upstream of two major sg RNA promoters are required for ef®cient sg RNA synthesis (Kim and Hemenway, 1999) . In RCNMV, an intermolecular RNA±RNA interaction is required for sg RNA synthesis (Sit et al., 1998) .