Some viruses, such as brome mosaic virus, initiate sg mRNA synthesis internally on the full-length minus strand RNA template (Miller et al., 1985) . Others, exempli®ed by red clover necrotic mosaic virus (RCNMV), may rely on premature termination of minus strand synthesis from the genomic RNA template, followed by the synthesis of sg plus strands from the truncated minus strand template (Sit et al., 1998) . Members of the order Nidovirales, which includes coronaviruses and arteriviruses, have evolved a third and unique mechanism, which employs discontinuous RNA synthesis for the generation of an extensive set of sg RNAs (reviewed by Brian and Spaan, 1997; Lai and Cavanagh, 1997; Snijder and Meulenberg, 1998) . Nidovirus sg mRNAs differ fundamentally from other viral sg RNAs in that they are not only 3¢-coterminal, but also 5¢-co-terminal with the genome ( Figure 1A) . A 5¢ common leader sequence of 65±221 nucleotides, derived from the 5¢ end of the genomic RNA, is attached to the 3¢ part of each sg RNA (thè mRNA body').