A Study of Changes in Ki67 Expression in People With Breast Cancer Receiving Endocrine Therapy Before Surgery

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