Galiellalactone is a fungal metabolite that inhibits IL-6-mediated JAK/STAT signal transduction in HepG2 cells (IC₅₀ = 0.25-0.5 μM). The selectivity of this compound is achieved by its ability to block the binding of activated STAT3 dimers to their DNA binding sites without affecting phosphorylation of the STAT3 transcription factor.  At 10-50 μM, galiellalactone exhibits dose-dependent growth inhibitory effects on prostate cancer stem cell-like cells expressing active STAT3, suggesting it may be a useful therapeutic approach to control JAK/STAT signaling.