Prospective Observational Study Evaluating Treatment Decision Impact of Prosigna® in Early Stage Breast Cancer Patients

Inclusion Criteria: * Surgically resected node-negative, estrogen receptor-positive, HER2-negative early-stage invasive breast cancer (T1-T2, N0, pN0 (i+), pN0 (mol+), M0) 1. Estrogen receptor status will be evaluated by Immunohistochemistry (IHC) and more than 1% of stained tumor cells will be considered positive. 2. HER2 status will be evaluated by IHC and/or by in-situ fluorescence hybridization (0 or 1+, or 2+ will be considered negative in the absence of in-situ fluorescence hybridization). * Postmenopausal females, which is defined as: 1. Natural Amenorrhea \> 12 months, regardless of age 2. Bilateral oophorectomy, regardless of age (the oophorectomy must have been carried out at least 4 weeks before entering the study) 3. Radiological castration with amenorrhea \> 3 months, regardless of age 4. Hysterectomy and postmenopausal blood levels of FSH/LH * Able to give informed consent * Eligible for treatment of breast cancer with adjuvant chemotherapy, as determined by the treating physician * ECOG performance status of 0 or 1 Exclusion Criteria: * Tumor specimen from core needle biopsy (CNB) * Tumor stage T3-T4 * Non-invasive breast cancer (e.g., Paget's disease, DCIS) * Tumors with nodes that are not N0, pN0 (i+), or pN0 (mol+) * Tumors that are estrogen receptor (ER) negative or HER2-positive * Have metastatic disease * Have received another genomic test for prognosis of early breast cancer (i.e., Oncotype Dx, Mammaprint, or BCI) * Unable to give informed consent * Unable to complete patient reported outcome surveys * Have contraindications for adjuvant chemotherapy, as determined by the treating physician o Age, performance status, significant comorbidities, etc. * ECOG performance status \> 1