Turkey Tail Mushroom for Treating Post-Menopausal Women With HER2-Negative ER-Positive Breast Cancer Undergoing Surgery

Inclusion Criteria: * Women ≥ 18 years of age * Histological confirmation of ER+, HER2- breast cancer, newly diagnosed and tissue available for central confirmation of Ki-67 measurement. Subjects are eligible if they have a Ki-67 of ≥10%. Ki-67 will be repeated for patients who have had this done externally. * Scheduled for definitive breast surgery * Detectable disease as defined by mammography, breast ultrasound of greater than 5mm in size. * NOTE: Tumor lesions in a previously irradiated area are not considered measurable disease; Disease that is measurable by physical examination only is not eligible * Post menopausal as defined by: * Self-reported last menstrual period greater than 12 months, or * Bilateral oophorectomy, or * Follicle stimulating hormone (FSH) \>20 mIU/mL, and estradiol level ≤ 20 pg/mL * Not taking aromatase inhibitor or a selective estrogen receptor modifier * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2 obtained ≤ 90 days prior to registration * Platelet count ≥ 100,000/mm\^3 (obtained ≤ 90 days prior to registration) * Absolute neutrophil count (ANC) ≥ 1,000/mm\^3 (obtained ≤ 90 days prior to registration) * Hemoglobin ≥ 11 g/dL (obtained ≤ 90 days prior to registration) * Serum transaminase \[alanine aminotransferase (ALT) or aspartate aminotransferase (AST)\] ≤ 1.2 x upper limit of normal (ULN) (obtained ≤ 90 days prior to registration) * Alkaline phosphatase ≤ 1.2 x ULN (obtained ≤ 90 days prior to registr…