Virus preparations that are used for IFN bioassays must not contain any type I IFN which may distort the results. In addition, the viruses should not induce IFN in infected reporter cells to assure that the effects measured are solely due to the exogenous IFN added. Both criteria are fulfilled for VSV*DG(Luc). The helper cells used to propagate the replicon particles are derived from BHK-21 cells, which are defective in the synthesis of type I IFN [32] . In addition, the VSVDG replicon does not induce type I IFN in infected reporter cells, because the VSV matrix protein efficiently blocks the nuclear export of cellular mRNA including the IFN mRNA [30, 33] .