Breast cancer is a histopathologically and molecularly heterogeneous disease, ranging from the more indolent luminal A tumors, which are generally estrogen receptor positive (ER+) and human epidermal growth factor receptor 2 (HER2)-negative, to the highly aggressive, basal-like triple-negative tumors, which are negative for the ER, progesterone receptor (PR) and HER2. Here, ESR1 is linked to breast cancer.