Patients with estrogen receptor α (ERα)-positive breast cancer are candidates for treatment with endocrine therapies such as the selective estrogen receptor modulator (SERM) tamoxifen (TAM), aromatase inhibitors (AIs) letrozole, anastrozole, or exemestane or the selective estrogen receptor downregulator (SERD), fulvestrant. Here, ESR1 is linked to breast carcinoma.