Various models have been put forward to explain the cotranscriptional fusion of non-contiguous parts of the nidovirus genome during sg RNA synthesis ( Figure 1B and C). Central to each of these models are short transcription-regulating sequences (TRSs), which are present both at the 3¢ end of the leader and at the 5¢ end of the sg RNA body regions in the genomic RNA. The TRS is copied into the mRNA and connects its leader and body part (Spaan et al., 1983; Lai et al., 1984) . Synthesis of sg mRNAs initially was proposed to be primed by free leader transcripts, which would base-pair to the complementary TRS regions in the full-length minus strand, and would be extended subsequently to make sg plus strands ( Figure 1B ; Baric et al., 1983 Baric et al., , 1985 . This model, however, was based on the report that sg minus strands were not present in coronavirus-infected cells (Lai et al., 1982) . The subsequent discovery of such molecules (Sethna et al., 1989) resulted in reconsideration of the initial`leader-primed transcription' model. Sawicki and Sawicki (1995) have proposed an alternative model ( Figure 1C ), in which the discontinuous step occurs during minus instead of plus strand RNA synthesis. In this model, minus strand synthesis would be attenuated after copying a body TRS from the plus strand template. Next, the nascent minus strand, with the TRS complement at its 3¢ end, would be transferred to the leader TRS and attach by means of TRS±TRS base pairing. RNA synthesis would be reinitiated to complete the sg minus strand by adding the complement of the genomic leader sequence. Subsequently, the sg minus strand would be used as template for sg mRNA synthesis, and the presence of the leader complement at its 3¢ end might allow the use of the same RNA signals that direct genome synthesis from the fulllength minus strand.