Oral Aromatase Inhibitors Modify the Gut Microbiome

The purpose of this research is to study the bacteria in participants' gut before receiving a medicine called an aromatase inhibitor for breast cancer and after participants receive medicine for breast cancer.

Inclusion Criteria: * Pathologic diagnosis in situ or invasive breast cancer for which initiating aromatase inhibitors therapy is recommended or initiating aromatase inhibitors for risk reduction. * ECOG performance status 0,1, 2, 3. * Age ≥ 18 years * HER2+ breast cancer is allowed if patients are on a monoclonal antibody therapy for which a change is not anticipated while on study (12 weeks) and they have been on the monoclonal antibody therapy for at least 4 weeks. * Ability to read, understand and evaluate study materials and willingness to sign a written informed consent document. Exclusion Criteria: * Have been on antibiotics within 4 weeks of enrollment. * Administered chemotherapy less than 4 weeks prior to enrollment. * Hormone replacement (defined as only estrogen and/or progesterone replacement) or anti-estrogen therapy within 4 weeks of starting aromatase inhibitors * Use of antibody drug conjugate (HER2+ breast cancer) * Male breast cancer * Any serious and/or unstable pre-existing medical, psychiatric, or other existing condition that would prevent compliance with the trial or consent process