We subjected idarubicin to further validation as an ISRE activator in assays of ISG expression and antiviral activity. For ISG expression, U3A (STAT1-null) and U3A-STAT1 cells were treated with a range of concentrations of idarubicin and IFN-b and then harvested for gene expression using quantitative realtime PCR assay. We found that idarubicin increased the expression of the antiviral gene 29,59-oligoadenylate synthetase 1 (OAS1), particularly with IFN-b treatment ( Figure 7 ). There was no effect of drug on ISG expression in STAT1-null U3A cells, indicating that the drug is specific for STAT1-dependent gene expression. We found similar results for the antiviral ISG guanylate-binding protein 1 (GBP1) and three other ISGs (MX1, PARP9, and IRF1) ( Figure 6 and data not shown).