The use of cytopathic viruses in conventional type I IFN bioassays makes appropriate biosafety measures necessary to reduce the risk of unwanted virus transmission and infection. In this regard, the VSV*DG(Luc) replicon particles can be regarded as biosafe. Since the modified VSV genome lacks the envelope glycoprotein (G) gene, VSV*DG(Luc) is propagated on helper cells providing the G glycoprotein in trans [23] . The replicon particles produced on these cells can run a single cycle of infection but cannot produce any infectious progeny [28] . Another aspect contributing to biosafety is that the RNA replicon replicates exclusively in the cytosol and does not produce cDNA intermediates. Thus, in the case of an accidental infection any risk of recombination with or integration into host chromosomal DNA can be excluded.