The structure for idarubicin shows characteristic features of an anthracycline antibiotic unrelated to any other antiviral compound in clinical use. To further validate the effect of idarubicin on ISRE activity, we tested three other anthracyclines (daunorubicin, doxorubicin, epirubicin) with very similar chemical structures to idarubicin. Each of these compounds also showed a capacity to significantly increase ISRE activity ( Figure 5B ). In addition, we found that the immune activators DMXAA (Vadimezan) and Imiquimod did not cause any increase in ISRE activity in the same concentration range ( Figure 5C ). These compounds appear to directly activate immune cells (including increased IFN production) [21, 22] . However, for the present work, we specifically studied non-hematopoietic cells since that population appears critical for STAT1-mediated defense against at least some types of viruses [11] . We also found a cytotoxic effect of idarubicin that is consistent with previous observations [23, 24] . Under the present conditions, the major effect of idarubicin on cell viability was detected at drug concentrations .3 mM, so that the ISRE-activating effect of idarubicin occurred at concentrations below those that cause a major effect on cell viability ( Figure 5D ). Nonetheless, idarubicin is best known as an anti-neoplastic agent that acts via DNA intercalation and topoisomerase II inhibition [25] . To determine whether the effect of idarubicin on ISRE activity is related to topoisomerase II inhibition, we tested three other potent topoisomerase inhibitors Etoposide, Hu-0331, and ICRF-193 up to concentrations known to cause topoisomerase II inhibition [26] [27] [28] . In contrast to idarubicin, these other compounds caused no significant increase in ISRE activity ( Figure 5E ). In fact, two of the topoisomerase inhibitors (Etoposide and Hu-0331) caused a decrease in ISRE activity in concert with cytotoxic effects at higher concentrations. These findings indicate that the capacity of idarubicin to activate the ISRE component of the IFN signaling pathway occurs independently of topoisomerase inhibition. Together, the findings provide evidence of idarubicin capacity to increase ISRE activity independent of the anti-neoplastic properties of the drug. We also observed that the effect of idarubin and the other anthracyclines on ISRE activity occurred at a lower concentration of drug when IFN-b was co-administered, particularly at the highest concentration of IFN-b (15 U/ml) ( Figure 5A ). For example, the EC 50 for idarubicin decreased as the concentration of IFN-b increased ( Table 2 ). These findings suggested that idarubicin might somehow interact with the IFN signaling pathway. In that regard, we also found that the effect of idarubicin on ISRE activity was observed under baseline conditions when there was no detectable production of endogenous IFN-b and no administration of exogenous IFN-b (data not shown and Figure 5A ). These results suggested that the effect of idarubicin on ISRE activity is independent of IFN production or action. Indeed, we also found that the effect of idarubicin on ISRE activity persisted without change during effective IFN-a/b receptor 2 (IFNAR2) blockade ( Figure 6 ). Together, the findings indicate that idarubicin causes an increase in ISRE activity independent of IFN production or IFN-IFN-receptor interaction and instead acts downstream of ligandreceptor binding in the IFN signaling pathway.