In particular, the selective estrogen receptor modulator (SERM) tamoxifen; the selective estrogen receptor downregulator (SERD) fulvestrant; or compounds that interfere with estrogen synthesis, such as aromatase inhibitors or ovarian suppression through luteinizing hormone-releasing hormone (LHRH) agonists, have become standard-of-care therapies in patients with HR+ BC in all clinical settings [11]. Here, ESR1 is linked to breast cancer.