Inclusion Criteria:

          -  PRE-SCREENING CRITERIA (STEP 0): History of locally advanced or metastatic breast
             cancer that is ERalpha negative or low (< 1% nuclear staining) and HER2 negative.

               -  Note: HER2 negative disease per 2018 American Society of Clinical
                  Oncology/College of American of Pathologists (ASCO/CAP) guidelines, one of the
                  following must apply:

                    -  0 or 1+ by immunohistochemistry (IHC) and not amplified by in situ
                       hybridization (ISH);

                    -  0 or 1+ by IHC and ISH not done;

                    -  2+ by IHC and ISH results are: < 6.0 HER2 signals/cell with HER2/CEP17 ratio
                       < 2.0;

                    -  IHC not done and not amplified by ISH.

          -  PRE-SCREENING CRITERIA (STEP 0): =< 3 prior chemotherapy regimens for treatment of
             metastatic breast cancer.

               -  Note: Prior use of monoclonal antibodies targeting PD1, PDL1 is allowed (if
                  administered as monotherapy it is not counted as a chemotherapy regimen).

          -  PRE-SCREENING CRITERIA (STEP 0): Eastern Cooperative Oncology Group (ECOG) performance
             status 0 or 1

          -  PRE-SCREENING CRITERIA (STEP 0): Willing to submit a biopsy specimen from locally
             recurrent or metastatic site (or primary if metastatic site not available) of breast
             cancer for ERbeta staining to Mayo Clinic Anatomic Pathology.

          -  PRE-REGISTRATION CRITERIA (STEP 1): Presence of moderate or strong nuclear ERbeta
             staining in > 25% of cells in specimen submitted during Pre-Screening Step.

          -  PRE-REGISTRATION CRITERIA (STEP 1): For patients who did not have a biopsy or lacking
             ERalpha, progesterone receptor (PR), and HER2 results from a locally advanced or
             metastatic site performed =< 12 months prior to Pre-Registration: Willing to undergo a
             standard of care biopsy of locally recurrent or metastatic breast cancer for ERalpha,
             PR, and HER2 as well as additional research cores.

          -  PRE-REGISTRATION CRITERIA (STEP 1): Measurable or non-measurable disease as defined by
             Response Evaluation Criteria in Solid Tumors (RECIST) criteria that will be assessed
             using imaging-based evaluations.

               -  Note: The tumor lesion biopsied during the pre-registration period is not
                  considered measurable disease nor a target lesion.

          -  PRE-REGISTRATION CRITERIA (STEP 1): If history of brain metastases must meet the
             following criteria:

               -  Patients with a history of brain metastases are eligible only if they are
                  asymptomatic and have stable disease for >= 3 months, including < 28 days of
                  prior to pre-registration.

               -  Not receiving steroids for brain metastases.

          -  PRE-REGISTRATION CRITERIA (STEP 1): ECOG performance status 0 or 1.

          -  PRE-REGISTRATION CRITERIA (STEP 1): =< 3 prior chemotherapy regimens for treatment of
             metastatic breast cancer.

               -  NOTE: Prior use of monoclonal antibodies targeting PD1, PDL1 is allowed.

          -  PRE-REGISTRATION CRITERIA (STEP 1): Women must be postmenopausal.

               -  NOTE: Postmenopausal status is verified by:

                    -  Prior bilateral surgical oophorectomy, or

                    -  Age >= 60 years, or

                    -  Age < 60 years with no menses for > 1 year with estradiol levels within
                       postmenopausal range, according to institutional standard.

          -  PRE-REGISTRATION CRITERIA (STEP 1): Able to swallow oral medications.

          -  PRE-REGISTRATION CRITERIA (STEP 1): Willingness to stop use of strong inducers or
             inhibitors of CYP3A4 prior to registration.

               -  NOTE: Use of strong inducers or inhibitors is allowed during pre-registration as
                  long as patient will complete course prior to registration.

          -  REGISTRATION CRITERIA (STEP 2): For patents who had a biopsy taken from a metastatic
             site =< 12 months prior to Pre-Registration: Confirmation from the local lab that the
             tumor from this biopsy was ERalpha negative (< 1% nuclear staining) and HER2 negative

          -  REGISTRATION CRITERIA (STEP 2): For patients who underwent a pre-registration biopsy:
             Histologic confirmation from local lab that tumor is ERalpha negative (< 1% nuclear
             staining), and HER2 negative

          -  REGISTRATION CRITERIA (STEP 2): Hemoglobin >= 8 g/dL (=< 14 days prior to
             registration).

          -  REGISTRATION CRITERIA (STEP 2): Platelet count >= 75,000/mm^3 (=< 14 days prior to
             registration).

          -  REGISTRATION CRITERIA (STEP 2): Creatinine =< 1.5 x upper limit of normal (ULN) (=< 14
             days prior to registration).

          -  REGISTRATION CRITERIA (STEP 2): Total bilirubin =< 1.5 x ULN (=< 14 days prior to
             registration).

          -  REGISTRATION CRITERIA (STEP 2): Aspartate aminotransferase/serum glutamic-oxaloacetic
             transaminase (AST/SGOT) =< 2.5 x ULN (=< 14 days prior to registration).

               -  For patients with liver metastasis =< 5 x ULN.

        Exclusion Criteria:

          -  PRE-REGISTRATION CRITERIA: Uncontrolled intercurrent illness including, but not
             limited to:

               -  Ongoing or active infection.

               -  Symptomatic congestive heart failure.

               -  Unstable angina pectoris.

               -  Uncontrolled symptomatic cardiac arrhythmia.

               -  Uncontrolled hypertension (defined as blood pressure > 160/90).

          -  PRE-REGISTRATION CRITERIA: Deep vein thrombosis / pulmonary embolism (DVT/PE) =< 12
             months prior to pre-registration.

               -  Note: Patients who are on anticoagulant therapy for maintenance are eligible as
                  long as the DVT and/or PE occurred > 6 months prior to pre-registration, and
                  there is no evidence for active thrombosis (either DVT or PE).

          -  PRE-REGISTRATION CRITERIA: Stroke =< 6 months prior to pre-registration.

          -  PRE-REGISTRATION CRITERIA: Two or more episodes of DVT and/or PE =< 5 years prior to
             pre-registration.

          -  PRE-REGISTRATION CRITERIA: Abnormal uterine bleeding =< 6 months prior to
             pre-registration

          -  PRE-REGISTRATION CRITERIA: History of coagulopathy.

          -  PRE-REGISTRATION CRITERIA: Other active second malignancy other than non-melanoma skin
             cancers within 3 years prior to pre-registration.

               -  NOTE: A second malignancy is not considered active if all treatment for that
                  malignancy is completed and the patient has been disease-free for >= 3 years
                  prior to pre-registration.

          -  REGISTRATION CRITERIA: None of the following therapies are allowed =< 14 days prior to
             registration.

               -  Chemotherapy.

               -  Immunotherapy.

               -  Biologic therapy.

               -  Hormonal therapy.

               -  Monoclonal antibodies.

               -  Anti-HER2 or other "targeted" (e.g. mTOR) therapy.

               -  Note: Any adverse events derived from these therapies must be =< grade 2 prior to
                  starting study therapy (exceptions for alopecia).