We believe that our present ®ndings strongly support the discontinuous minus strand extension model. Indeed, the fact that a complete body TRS can be copied into the sg RNA is very dif®cult to reconcile with the alternative model, in which sg RNA synthesis from the genomic minus strand template is primed by free plus strand leader transcripts that contain the leader TRS at their 3¢ end ( Figure 1B) . To explain the presence of a complete copy of the body TRS in the sg mRNA in this model, one would have to assume that a 3¢±5¢ exonuclease activity trims back the free leader transcript prior to its extension into an sg mRNA (Baker and Lai, 1990) . Note that there would not be a single base pair left to hold these`trimmed' leader molecules on the template. Such an enzymatic activity, which is unprecedented in +RNA viruses, exists in yeast retrotransposon Ty5 (Ke et al., 1999) , in which reverse transcription is primed by an internal region in a tRNA. However, in this system, it is not a part of the duplex that is removed, but the single-stranded 3¢ tail of the tRNA, which cannot base-pair with the Ty5 RNA.