Using the position 1 mutants described above, we could answer this question ( Figure 5) . The most striking result was that mRNA7 of mutants BU 1 A, BU 1 G and LU 1 C contained exclusively the body TRS-derived nucleotide at position +1. Thus, for these mutants, the crossover site could be mapped precisely between TRS nucleotide positions ±1 and +1, meaning that the complete leader± body junction sequence in an EAV sg mRNA can be body TRS derived. On the other hand, sg RNAs from mutants LU 1 A, BU 1 C and LU 1 G contained mixed populations of leader TRS-and body TRS-derived nucleotides at position +1 ( Figure 5 ): A and U for LU 1 A, C and U for BU 1 C, and G and U for LU 1 G. Remarkably, this pattern correlated with the relative amounts of sg mRNA7 produced by these mutants (Figure 3 ). Mutants that produced populations of sg RNAs that were mixed with respect to the origin of the nucleotide at position +1 of the leader±body junction had lost RNA7 synthesis almost completely. On the other hand, mutants that contained exclusively the body nucleotide at position +1 retained higher levels of RNA7 synthesis. This observation may be explained as follows: in the wild-type situation, the large majority of the crossovers probably occur between positions ±1 and +1, leading to a body TRS-derived nucleotide at position +1 in the sg RNA; however, a low number of crossovers take place between nucleotides +1 and +2, resulting in a leader TRS-derived nucleotide at position +1. Mutants in which almost all sg RNA synthesis is blocked by a substitution at position +1 may somehow be de®cient in the crossover between ±1 and +1, but may have retained the ability for crossovers between +1 and +2, which were detected by sequence analysis. Conversely, in position +1 mutants that retain reasonable sg RNA synthesis, most crossovers occur between positions ±1 and +1, and they obscure the minority of crossovers between +1 and +2 in the sequencing electropherogram. Alternatively, position +1 TRS mutations that strongly interfere with sg RNA synthesis may force a shift of the crossover site in the remaining molecules.