We used the automated ISRE-activity assay to perform a screen of a 2240 chemical compound library. This library consisted of 2160 compounds from the Johns Hopkins Clinical Compound Library (JHCCL) of FDA approved or approvable drugs [19, 20] . In addition, we included 33 compounds from the Screen-Well Phosphatase Inhibitor Library based on the observation that the improvement in IFN signaling in STAT1-CC-expressing cells correlated with prolonged phosphorylation of STAT1 and STAT2 [13] . Each compound was tested at 4 different concentrations (0.24, 1.2, 6 and 30 mM) and simultaneous treatment with IFN-b at 5 U/ml, the concentration at the initial inflexion of the IFN concentration-response curve (as shown in Figure 2F ). These treatment conditions were duplicated on a second plate. Each assay plate also contained control wells containing a range of IFN- Figure 1 . Scheme for IFN signal transduction. Type I IFN signaling starts by activation of the IFN-a/b receptor (IFNAR) and subsequent activation of the IFNAR1-associated TYK2 and IFNAR2-associated JAK1, with consequent recruitment of STAT2. Phosphorylation of STAT2 enables reruitment of STAT1 and release of the phosphorylated STAT1-STAT2 heterodimer bound to IRF-9. This complex binds to the IFN stimulated response element (ISRE) and in concert with recruited transcriptional co-activators such as p300/CBP then drives IFN-stimulated gene (ISG) transcription. doi:10.1371/journal.pone.0036594.g001 b concentrations (0-200 U/ml) in quadruplicate ( Figure 3A ). This arrangement achieved excellent signal reproducibility between duplicate compound plates as well as signal consistency through the full screening run of 56 assay plates (28 duplicate pairs) ( Figure 3B , C).