ESR1 and breast cancer: Recently, many investigators have explored the potential of applying hormone therapy on GC, particularly targeting estrogen and estrogen receptor (ER).[6–9] Hormone therapy targeting estrogen consists of an aromatase inhibitor, an ER inhibitor (tamoxifen), and a selective ER degrader (fulvestrant).[10] Aromatase inhibitor, such as letrozole, exemestane, or anastrozole, are applied for the treatment of breast cancer in postmenopausal women.