Thus, although the number of patients was limited, and contrary to the data available in breast cancer, our study highlights several elements suggesting that the emergence of ESR1 mutations is not a privileged mechanism of hormone resistance in metastatic endometrial cancer: 1) Among the eight patients (36.4%) exposed to hormone therapy for at least 6 months and who progressed, only one (4.5%) had a mutation before starting hormone therapy, which disappeared upon progression. The gene discussed is ESR1; the disease is breast cancer.