ER covalent antagonist-1 (Compound 39D) is the antagonist for estrogen receptor α ( ERα ). ER covalent antagonist-1 inhibits the proliferation of ERα-positive cell MCF-7 with an IC 50 of 0.98 μM, arrests the cell cycle at G0/G1 phase, and induces apoptosis. ER covalent antagonist-1 exhibits antitumor efficacy in mouse model.