ESR1 and breast cancer: Based on gene expression profiles and receptor status (estrogen receptor [ER], progesterone receptor [PR], human epidermal growth factor receptor 2 [HER2]) and on proliferation status as assessed by Ki67, Perou et al. subdivided breast cancer into four main clinical subtypes [17]: luminal A (ER+/PR+), luminal B (ER+/PR+/HER2−/+/Ki67+), HER2 overexpressing (ER−/PR−/HER+), and basal-like/triple-negative (TN) (ER−/PR−/HER2−).