OBHSA is a selective estrogen receptor ( ERα ) degrader. OBHSA blocks the cell cycle by degrading cyclin D1, thereby overcoming Tamoxifen resistance. OBHSA also triggers excessive activation of the unfolded protein response (UPR) by inducing an increase in intracellular reactive oxygen species , ultimately leading to cell apoptosis. In addition, OBHSA can also be used as an ERα ligand to synthesize PROTAC degraders.