NCT03332797 (accessed on date 2021 December) is evaluating the safety, pharmacokinetic (PK), pharmacodynamic (PD) activity, and preliminary anti-tumor activity of GDC-9545 as a single agent and in combination with palbociclib and/or luteinizing hormone−releasing hormone (LHRH) agonist in participants with advanced or metastatic ER +, HER2 -, BC. This evidence concerns the gene ESR1 and breast cancer.