A Study of Clinical Outcomes for the NanoString® Technologies Prosigna™ Gene Signature Assay

Inclusion Criteria: * Resected node-negative, estrogen-receptor-positive, HER2-negative early- stage invasive breast cancer (T1-T2, N0, pN0 (i+), pN0 (mol+), M0) * Estrogen receptor status will be evaluated by Immunohistochemistry (IHC) and more than 1% of stained tumor cells will be considered positive. * HER2 status will be evaluated by IHC (0 or 1+, or 2+ will be considered negative) and by in-situ fluorescence hybridization. * Postmenopausal women defined as: * Natural Amenorrhea \> 12 months, regardless of age * Bilateral oophorectomy, regardless of age (the oophorectomy must have been carried out at least 4 weeks before entering the study) * Radiological castration with amenorrhea \> 3 months, regardless of age * Hysterectomy and postmenopausal blood levels * Able to give consent * Eligible for treatment of breast cancer with adjuvant chemotherapy * ECOG performance status of 0 or 1 Exclusion Criteria: * Tumor size 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 negative or HER2 positive * Have metastatic disease * Unable to give informed consent * Unable to complete patient reported outcome surveys * Have contraindications for adjuvant chemotherapy * Age, performance status, significant comorbidities * ECOG performance status \> 1