• Subject must be female age \>= 18 years.

• Postmenopausal as defined by at least one of the following:

‣ Age \>= 60 years;

⁃ Age \< 60 years and cessation of regular menses for at least 12 consecutive months with no alternative pathological or physiological cause; or serum estradiol and follicle stimulating hormone levels within the local laboratory's normal reference range for postmenopausal females;

⁃ Documented bilateral oophorectomy.

• Histologically confirmed diagnosis of carcinoma of the breast, clinical stage I-III.

• Minimum tumor diameter of 1.0 cm as assessed by palpation or imaging (e.g., ultrasound) and with enough tumor left after the initial diagnostic biopsy to provide tumor tissue during the planned tumor resection.

• Candidate for surgical resection.

• ER+ breast cancer (\> 1% positive stained cells) based on the most recent tumor biopsy documented by a local laboratory or medical record.

• Ki67 \>= 10%.

• HER2-negative breast cancer based on the most recent tumor biopsy and documented by a local laboratory or medical record. HER2-negative tumor is defined per American Society of Clinical Oncology and the College of American Pathologists guidelines, 2018.

• Stated willingness to comply with all study procedures and availability for the duration of the study.

• Prior use of hormone contraceptives and replacement therapy is allowed (e.g., estrogen and/or progestin) if discontinued \> 6 months prior to diagnosis. Vaginal preparations are allowed.

• Ability to take oral medication and be willing to adhere to the study intervention.