ESR1 and breast cancer: In the competent assay, estrogen receptor warhead (4-OHT) and the IAP antagonist (LCL161 derivative), when treated together, no decrease in the ER-α protein level was observed in MCF-7 breast cancer cells (IC50 = 15.6 nM), suggesting SNIPER(ER)-87 as an ideal example of event-driven pharmacology.