Hormone Receptor Positive Metastatic Breast Cancer (HR+ mBC) BIIB021 Plus Aromasin Schedule Finding

Inclusion Criteria: * Age 18 years of age * Must have histologically or cytologically confirmed estrogen receptor- positive or progesterone receptor-positive, incurable, locally advanced, or metastatic breast cancer. * Must have disease progression during treatment with a nonsteroidal AI for locally advanced or metastatic disease, or relapse during treatment or within 12 months of discontinuation of treatment in the adjuvant setting. * Must be a postmenopausal female. * Must have measurable or evaluable disease. * Measurable disease is defined as \>=1 lesion with a diameter of \>=10 mm * Evaluable disease is defined as bone lesions evaluable by plain X ray, CT scan, or MRI. Lesions identified only by radionuclide bone scan are not allowed. * One prior chemotherapy regimen for advanced mBC is allowed. * Prior radiotherapy is allowed. * Must be able to swallow and retain oral medication. * ECOG performance status of \<=2 * Required laboratory values * Plasma cortisol and adrenocorticotropic hormone (ACTH) levels that are not suggestive of adrenal insufficiency unless on replacement therapy for known adrenal insufficiency. Exclusion Criteria: * HER2 overexpressing tumor. * History of central nervous system (CNS) metastasis. * Previous treatment with exemestane or treatment with an Hsp90 inhibitor. * Use of proton pump inhibitors. * Known history of or positive test result for hepatitis B or C or HIV. * History of gastrectomy or major surgery to small intestine.