ESR1 and breast cancer: Within the setting of early-stage estrogen receptor-positive, human epidermal growth factor receptor-2-negative (ER+/HER2− ) breast cancer, the Oncotype DX© Recurrence Score (RS) 21-gene expression assay (commercially available at Genomic Health Inc., Redwood City, California, United States) is used to predict distant disease recurrence following treatment with curative intent and to estimate prognosis [8, 9].