ESR1 and breast cancer: Because E2 can induce apoptosis, E2 was successfully usedin high concentrations to treat ERα-positive breast cancer inpostmenopausal women11 before tamoxifenwas used as an antiestrogen drug.12 Afterthe development of antiestrogen drugs, E2 has been exploited as analternative drug to overcome antiestrogen resistance in the treatmentof ER-positive breast cancer.13,14 Although the resultsare not conclusive, there is potential for E2 as a viable therapyfor breast cancer patients.15