An Observational Study of Women With Breast Cancer Examining the Effect of Endocrine Therapy on Aging

The goal of this pilot observational study is to learn about the feasibility of collecting patient-reported data and stool and blood samples from patients age 65 and older treated with aromatase inhibitor therapy for breast cancer. Participants will be treated with standard of care aromatase inhibitor therapy and will undergo serial phlebotomy, complete patient-reported questionnaires, and submit serial stool samples. The main exploratory translational questions it aims to obtain preliminary data for are: * What are the effects of aromatase inhibitor therapy on biomarkers of aging? * What are the effects of aromatase inhibitor therapy on the microbiome? These data will be used for the development of future definitive studies.