Abstract: The present invention relates to a novel mechanism to control RNA virus replication and gene expression using a conditional protease approach using a specific protease inhibitor for regulation. More specifically it relates to a single-stranded RNA virus, preferably of the order Mononegavirales, comprising a polynucleotide sequence encoding at least one protein essential for viral transcription and/or replication, a protease and a cleavage site for said protease. The protease can be inhibited using a protease inhibitor and hence the protease and the cleavage site for said protease form a regulatable switch. By changing the insertion site of the regulatable switch from an INTRA- to an INTER-molecular location in the at least one protein essential for viral transcription and/or replication, the effect of the protease inhibitor can be altered from an ON switch to an OFF switch. RNA virus may further encode a heterologous protein, the expression of which is then regulated by regulating viral activity.