The transcription from 25CAT RNA was strongly inhibited by the dominant-negative mutant of hnRNP A1, as shown by CAT assays ( Figure 6A ). In addition, the replication of the naturally occurring A59 DI RNA and the arti®cial DIssE RNA was completely abolished ( Figure 6B) . Surprisingly, the replication of MHV DI RNAs suffered a stronger inhibition by the dominantnegative mutant of hnRNP A1 than the synthesis of MHV genomic and subgenomic RNAs, suggesting that DI RNA replication may be more dependent on hnRNP A1. Although DI RNAs contain all of the cis-acting replication signals that are essential for their replication in normal cells (Kim and Makino, 1995) , the small size of DI RNA may cause it to require more hnRNP A1 to maintain a critical RNA structure. It has been shown that different DI RNAs require different cis-acting signals for RNA replication (Kim and Makino, 1995) .