Molecular pathology evaluation, typically by immunohistochemistry (IHC), identifies expression levels of several key biomarkers: estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor-2 (HER2), and the proliferation marker Ki67; these markers are critical for disease subtyping, prognosis, and for selecting therapies to which tumor subtypes are most likely to respond7–9. The gene discussed is ESR1; the disease is neoplasm.