There is an accumulating body of evidence signifying the importance of cellular factors in RNA synthesis of RNA viruses (reviewed by Lai, 1998) . Previous studies have shown that hnRNP A1 binds to the cis-acting sequences of MHV template RNA and that this interaction correlates with the transcription ef®ciency of viral RNA in vivo (Zhang and Lai, 1995; Li et al., 1997) . In addition, hnRNP A1 is also implicated in viral RNA replication by the recent ®nding that hnRNP A1 interacts with the 3¢-ends of both positive-and negative-strand MHV RNA (P.Huang and M.M.C.Lai, unpublished results). However, hnRNP A1 modulates cytoplasmic viral RNA synthesis the functional importance of hnRNP A1 in viral RNA synthesis has so far not been directly demonstrated. In the present study, we established that MHV RNA transcription and replication were enhanced by overexpression of the wt hnRNP A1 protein, but inhibited by expression of a dominant-negative hnRNP A1 mutant in DBT cell lines.