The 21-gene Oncotype DX Breast Recurrence Score® assay is a prospectively validated prognosticator and predictor of chemotherapy (CT) benefit, and is used to guide adjuvant treatment decisions in patients with early stage hormone receptor-positive human epidermal growth factor receptor 2 negative (HER2‒) breast cancer (BC)1–7. This evidence concerns the gene ERBB2 and breast carcinoma.