ESR1 and breast cancer: For metastatic ER+/HER2− breast cancer without visceral crisis (which would be indicated by severe organ dysfunction assessed by signs, symptoms or laboratory studies, resulting from the rapid progression of cancer), endocrine therapy with aromatase inhibitors (AIs), (such as letrozole), tamoxifen, or fulvestrant, is the therapeutic cornerstone (Palmieri et al. 2014).