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. This evidence concerns the gene ESR1 and breast cancer.