In luminal breast cancer models (BT-474, MCF-7), OC (IC50: 20–84 μM) reduced total ERα protein levels by 60% via proteasomal degradation, while competitively inhibiting 17β-estradiol (E2) binding to the LBD with a dissociation constant (Kd) of 0.8 μM—comparable to tamoxifen [77]. This evidence concerns the gene ESR1 and breast cancer.