(-)-Galiellalactone (GL) and its analogues can block the STAT3 signaling pathway. (A) The chemical structure of (-)-galiellalactone (GL), SG-1709, and SG-1721. (B) Detection levels of p-STAT3 and STAT3 in seven human breast cancer cell lines were analyzed by Western blot analysis. (C) BT-549, BT-20, and MDA–MB-468 cells were treated with 10 μM of GL, SG-1709, and SG-1721 for 4 h, and Western blot analysis was done for different proteins. (D) MDA–MB-468 cells as described above for various time intervals and Western blot analysis was done. (E) MDA–MB-468 cells were treated with concentrations of GL, SG-1709, and SG-1721 for 4 h, and Western blot analysis was done. (F) MDA–MB-468 cells were treated as described above in panel D and E, and electrophoretic mobility shift assay was done. (G) MDA–MB-468 cells were treated with 10 μM of GL, SG-1709, and SG-1721 for 4 h, and immunocytochemistry was done.