The researchers are doing this study to look at changes in Ki67 expression after at least 2 weeks of endocrine therapy in people with ER+/HER2- breast cancer undergoing cancer removal surgery. Participants will receive the endocrine therapy before their surgery. The researchers will look at how changes in Ki67 expression compare between participants who are carriers of the BRCA2 mutation and participants who are noncarriers of the BRCA2 mutation.

Inclusion Criteria:

• Postmenopausal women aged ≥18 years with biopsy-proven stage I-III invasive breast cancer that is ER+/HER2-
• Scheduled to undergo upfront surgery
• Eligible for genetic testing in accordance with National Comprehensive Cancer Network guidelines(BRCA2 arm only)

Exclusion Criteria:

• History of breast cancer
• Receipt of ET for risk reduction in the previous 3 months
• Stage IV disease at presentation
• Scheduled to undergo neoadjuvant systemic chemotherapy
• Pregnant

Breast Cancer

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.