ESR1 and breast cancer: [18F]FluoroEstradiol positron emission tomography (FES-PET), [18F]FluoroDeoxyGlucose (FDG) PET, and [18F]FluoroThymidine (FLT) PET were investigated as early pharmacodynamic, tumor metabolism, and tumor proliferation imaging biomarkers, respectively, in mice bearing subcutaneous MCF7-Y537S mutant ERα+ breast cancer model treated with the SERD agent SAR439859.