Inclusion Criteria:  Advanced breast cancer patients, i.e. stage 3 or 4 diseases. Aged 18 or older at the time of breast cancer diagnosis. Histopathologic review confirming diagnosis of ER positive breast cancer. Male patients, although rare, can participate. Patient must have given informed consent. ECOG performance status 0-3. ER must be positive in at least 10% of tumor cells in either the core biopsy or a therapeutic surgical resection. Any HER2-neu status. Patient eligible for and willing to receive systemic treatment.  Exclusion Criteria:  Patients who are pregnant or nursing. Patients unable to tolerate PET/CT for 30 minutes. Patients taking anti-ER hormone therapy that can pharmacologically blockade estrogen receptors (ex: Tamoxifen, Fulvestrant). Patients can undergo 4FMFES-PET if anti-ER therapy was interrupted at least 8 weeks prior imaging. Aromatase inhibitors and LHRH analogs are not expected to interfere with 4FMFES uptake and are thus permitted to be taken prior and during this study.