ESR1 and breast carcinoma: Currently, the selection of individualized therapy for patients with ER-positive, human epidermal growth factor receptor type 2 (HER2)-negative early stage breast cancers is guided by the OncotypeDX gene expression assay (Genomic Health Inc., Redwood City, CA)4, which produces a recurrence score between 0–100 to predict the likelihood of disease recurrence with tamoxifen and improves the oncologist’s ability to estimate benefit from adjuvant hormonal and chemotherapy5, 6.