ESR1 and breast cancer: Standard biomarker testing for breast cancer includes assessment of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression by immunohistochemistry (IHC) and HER2 amplification by fluorescence in situ hybridization (FISH) to determine patient eligibility for hormonal and anti-HER2 therapy [4–6].