Inclusion Criteria:

- ER+ Her2- breast cancer

- Metastatic

- At least 18 years of age

- Candidate for fulvestrant therapy - patients who have started fulvestrant may enter this trial if within 3 months of starting fulvestrant

- Measurable or Evaluable by RECIST 1.1

- PS 0-2

- Able to swallow study drug and comply with study requirements

- Tumor available for fresh biopsy (two biopsies - pretreatment as regards enzalutamide, and during treatment @4 weeks). (The patient will be asked if they would be willing to provide a third biopsy at time of progression)

- Patients may already have gotten the loading doses of fulvestrant and/or ovarian suppression

- If patient is pre- or peri- menopausal, then will need to have concurrent ovarian suppression.

- ANC >1000/uL, platelets >75,000/uL at screening visit

- Total bilirubin < 1.5 times upper limit of normal (ULN) at the screening visit unless an alternate nonmalignant etiology exists (eg, Gilbert's disease)

- Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) < 3 times ULN or < 5 times ULN if patient has documented liver metastases

- Creatinine < 1.5 times ULN

- INR < 1.5 times ULN, or if on warfarin, can safely transition off for biopsy

- Willing to donate blood for research at 4 time points

- Written informed consent obtained prior to biopsies and blood samples

Exclusion Criteria:

- Current or previously treated brain or leptomeningeal metastases

- History of seizures

- Prior treatment with an anti-androgen (abiraterone, ARN-509, bicalutamide, enzalutamide, ODM-201, TAK-448, TAK-683, TAK-700, VT-464).

- Systemic estrogens or androgens within 14 days before initiating therapy. Vaginal estrogens are allowed if necessary for patient comfort.