ClinicalTrials.gov identifier NCT04505826 is a phase 1 dose escalation and dose expansion and phase 2 monotherapy trial to assess the dose-limiting toxicity (DLT), maximum tolerated dose (MTD), and/or recommended phase 2 dose, to analyze the safety and pharmacokinetic profile, and to evaluate the preliminary anti-tumor activity of palazestrant as a single agent in adult with ER+/HER2− metastatic breast cancer or locally advanced breast cancer with and without ESR1 mutation. Here, ESR1 is linked to breast cancer.