ESR1 and breast carcinoma: Histopathologic analysis of breast cancers (BCs) for estrogen receptor (ER) and progesterone receptor (PgR) content, human epidermal growth factor receptor 2 (HER2) gene amplification, and Nottingham histologic grade (NHG) are the mainstays of current clinical practice.1 Increasingly, assessment of the proliferation antigen Ki67 is clinically recommended.2 These five biomarkers carry prognostic and predictive information and are used in combination with other clinicopathological factors for risk stratification and therapy selection.1