ESR1 and breast cancer: Breast cancer can be classified into three immunohistochemistry (IHC)-based molecular subtypes [8]: estrogen receptor (ER) positive (and human epidermal growth factor receptor 2 [HER2] negative, independent of progesterone receptor [PR] status), HER2 positive (independent of ER and PR status), and “triple negative” (ER negative, PR negative, and HER2 negative).