The reporter plasmids pGL3-Control and phRG-TK were purchased from Promega. pGL2-CVX contains two repeats of the IFN-responsive GAS (gamma activated sequence) elements (GATCTGGATTTAGAGTAATATGAAACTGAAAGTACTTCG) of the guanylate-binding protein (GBP) gene in front of a CMV minimal promoter and was kindly provided by Ute Pägelow and Mario Köster from the Helmholtz-Zentrum für Infektionsforschung.