Casticin (0.2-1.0 μM) dose-dependently inhibits the proliferation of KB cells, with an IC50 of 0.23 μM on day 3, while shows no significant inhibition on 3T3 Swiss Albino and TIG-103 cells. Casticin (0.6 μM) alters spindle morphology with partial mitotic spindle breakdown or with disordered spindles. Casticin (0-40 μM) dose-dependently inhibits the proliferation of LX2 cells. Casticin (40 μM) suppresses L02 cells proliferation and induces apoptosis. Casticin inhibits fibrotic effects of TGF-β1 on ECM deposition in LX2 cells by evaluating the mRNA levels of TGF-β, collagen α1(I), MMP-2, MMP-9, TIMP-1 and TIMP-2. Casticin (0-8 μM) reduces the viability of 786-O, YD-8, and HN-9 cells, but shows no significant effect on that of the normal HEL 299 cells. Casticin (5 μM) increases cleavage caspase-3 and PPAR, diminishes the levels of B-cell lymphoma-extra large (Bcl-xl), Bcl-2, IAP-1/-2, vascular endothelial growth factor (VEGF), matrix metallopeptidase 9 (MMP-9), and cyclooxygenase 2 (COX-2) proteins in 786-O, YD-8, and HN-9 cells. Casticin (5 μM) also promotes apoptotic cell death, inhibits constitutively active STAT3 in tumor cells, modulates STAT3 activation by altering the activity of upstream STAT3 regulators, and abrogates IL-6-induced STAT3 activation. In addition, Casticin (2.5 μM) enhances the effect of ionizing radiation in 786-O cells and potentiates the therapeutic effect of radiotherapy.