Antiestrogen treatments for patients with ER+ or PR+ breast cancer inhibit ER by antagonizing estrogen ligand binding to ER (tamoxifen and other selective estrogen receptor modulators, SERMs), inhibiting dimerization and downregulating ER (fulvestrant and other selective estrogen receptor downregulators, SERDs), or blocking estrogen production (aromatase inhibitors, AIs; letrozole, anastrozole, exemestane). Here, ESR1 is linked to breast cancer.