The results of our present study strongly suggest that nidovirus discontinuous sg RNA synthesis can be considered a special case of high-frequency similarity-assisted RNA recombination. While the only obvious function of the leader TRS is to ensure the ®delity of the strand transfer by base pairing with the 3¢ end of the nascent strand, the body TRS in the donor template indeed has additional, sequence-speci®c functions. One of these functions apparently is to pause (or terminate) nascent strand synthesis and thereby provide the opportunity for strand transfer. In addition, body TRS-derived nucleotides may play a role in the reinitiation of nascent strand synthesis on the acceptor template. Given the compact nature of the EAV TRS, it is quite possible that some nucleotides ful®l multiple tasks.