In contrast to our ®ndings with the body TRS mutants, we did not obtain leader TRS mutations that appeared to determine the level of sg RNA7 synthesis of the corresponding double mutant (Figure 3) . Thus, effects of mutations in the leader TRS were not`dominant' over the duplex-restoring effects of the double mutations, suggesting that they only affected duplex formation. This indicated that the leader TRS probably does not have an additional, sequence-speci®c function in sg RNA synthesis in addition to its participation in TRS±TRS base pairing. The fact that single leader TRS mutations at all six Nidovirus discontinuous subgenomic RNA synthesis positions severely repressed RNA7 synthesis indicated that base pairing of every TRS nucleotide contributes to sg RNA production. In this respect, it was signi®cant that the two leader TRS mutants with the highest RNA7 levels, LA 3 G and LA 4 G, can form G±U base pairs to maintain the duplex.