(A) Comparison of treatment with 10 µM inhibitor 4 (24 h) on STAT3 phosphorylation in wild-type (WT) MDA-MB-468 cells and MDA-MB-468 E2F1 knockdown cells (E2F1 siRNA). (B) Effect on MDA-MB-468 cell viability of E2F1 knockdown alone (siRNA) and with 10 µM inhibitor 4 (72 h) (siRNA + 4). (C) Effect on MDA-MB-468 cell viability of treatment (10 µM, 72 h) with STAT3 phosphorylation inhibitor cryptotanshinone (CPT), CPT + inhibitor 4 co-treatment, and treatment with 4 alone compared to vehicle treated control (Cntrl). (D) Proposed CDK8 inhibitor mechanism. N.S. p > 0.05 (not significant), ** p < 0.005 (significant), *** p < 0.001 (very significant).