In an attempt to validate whether the cytotoxic effects of dronedarone are mediated through THRα1, or even THRα, siRNA to knockdown of both targets was pursued in a variety of breast cancer cell lines representing the different intrinsic receptor subtype and molecular backgrounds, including well-characterized luminal A, luminal B, Her2+, and triple negative (basal-like) breast cancer cell lines, 600 MPE, HCC1954, MDA-MB-231, MDA-MB-468, SUM159 PT, and T47D. This evidence concerns the gene THRA and breast cancer.