Sequence requirements for RNA strand transfer during nidovirus discontinuous subgenomic RNA synthesis The EMBO Journal Vol. 20 No. 24 pp. 7220±7228, 2001 Using site-directed mutagenesis of TRSs of the arterivirus equine arteritis virus (EAV), we have shown previously that base pairing between the sense leader TRS and antisense body TRSs is crucial for sg mRNA synthesis (van Marle et al., 1999a) . However, base pairing is only one step of the nascent strand transfer process and is essential in both models outlined in Figure 1 . The EAV genomic RNA contains several sequences that match the leader TRS precisely, but nevertheless are not used for sg RNA synthesis (den Boon et al., 1996; Pasternak et al., 2000) . This suggests that leader±body TRS similarity alone is, though necessary, not suf®cient for the strand transfer to occur.