Removal of the TRS at the 3¢ end of the nidovirus leader, which has already base paired with the template, would be very energetically unfavourable for the RdRp. Instead of starting elongation using the intact and properly positioned leader as a primer, it would have to disrupt the newly formed duplex, degrade part of the leader RNA and then reinitiate polymerization, without any base pairing between primer and template. It has been shown that in¯uenza virus transcription does not require a sequence match between the (cellular) RNA primer and the (viral) template (Plotch et al., 1981) . However, if in the nidovirus system the`trimmed' leader RNA could also be ®xed on the template solely by RNA±protein interactions, the targeting of the nascent strand by TRS base pairing would be extremely puzzling.