ESR1 and breast cancer: This molecular stratification complements the pathological classifiers of breast cancer, namely estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), and is preserved in the established human breast cancer cell lines (Holliday and Speirs, 2011; Parker et al., 2009; Prat and Perou, 2011).