RecruitingProcedureA Study of Changes in Ki67 Expression in People With Breast Cancer Receiving Endocrine Therapy Before SurgeryThe 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.