The cornerstone of treatment for patients with endocrine-sensitive advanced HR+/HER2− breast cancer is endocrine therapy (ET), which consists of selective estrogen receptor modulators (SERM: tamoxifen), aromatase inhibitors (AIs: letrozole, anastrozole, exemestane) or selective estrogen receptor degraders (SERDs: fulvestrant)2,3. Here, ESR1 is linked to breast cancer.