In the current AMEERA-1 study, among postmenopausal women with metastatic ER+/HER2− breast cancer treated with amcenestrant ≥20 mg QD as monotherapy, we observed an absence of DLTs without reaching the maximum tolerated dose (MTD), a favorable safety profile, and a strong pharmacokinetic/pharmacodynamic relationship between plasma amcenestrant concentration and saturation of ER occupancy in multiple tumor sites by 18F-FES PET/CT imaging at doses ≥150 mg as well as evidence of antitumor activity. Here, ESR1 is linked to breast carcinoma.