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.

Primary Objective: To describe the microbiome shift of gut microbiome from baseline after 4-weeks and 12-weeks of oral aromatase inhibitor treatment.

Secondary Objective

• To correlate gut microbiome populations with circulating sex hormone metabolite concentrations at baseline and 12 weeks.
• To describe the correlation of change in the microbiome populations with change in circulating sex hormone metabolite concentrations from baseline to 12 weeks.

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

Women of all races and ethnicity who meet the eligibility criteria are eligible for this trial.