ESR1 and breast cancer: Breast cancer can be divided into four major subtypes on the basis of HR and HER2 testing: those that are HR + (ER + and/or PR+) but HER2-, those that are HR + and HER2+, those that are HR- and HER2+, and those that are “triple negative” for ER, PR, and HER2 [7,8,10-12].