Studies have shown that the use of tamoxifen postoperatively can lead to disease progression in ESS patients, as well as the development of ESS in breast cancer patients.22, 23, 36 In 2014, the European Society for Medical Oncology (ESMO) guidelines noted that tamoxifen and HRT is contraindicated drugs of ESS treatment.37 Yet, fulvestrant, the pure estrogen alpha receptor inhibitor, has been reported successfully reducing the tumor diameter in an LGESS (ER+) patient who refused surgery but developed multiple metastases after treated with progesterone and anastrozole.38 The gene discussed is ESR1; the disease is endometrioid stromal sarcoma.