ESR1 and breast cancer: To determine whether triptolide (Fig. 1A) exhibited toxic effects on breast cancer cells, highly metastatic MDA-MB-231, HER2-positive BT-474 and ER-positive MCF7 cells were treated with DMSO only (0 nM triptolide) or triptolide at a concentration of 10, 25 or 50 nM.