We next investigated whether the mutant hnRNP A1 is de®cient in the interaction with any other cellular proteins in this RNA±protein complex. We labeled proteins in MHV-infected cells or mock-infected cells at different time points after infection and immunoprecipitated with the anti-Flag antibody. Signi®cantly, a cellular protein of 250 kDa was shown to be associated only with the wt hnRNP A1, but not the mutant hnRNP A1 ( Figure 7D ), suggesting that hnRNP A1 binds to this protein through its C-terminal domain. We propose that this cellular protein is another important component of the MHV RNA transcription/replication complex.