18F-Fluorodeoxyglucose (FDG) and 18F-Fluorestradiol (FES) have been FDA approved for measuring tumor glycolytic activity and estrogen receptor (ER) uptake, respectively, in clinical positron emission tomography (PET) imaging for patients with hormone-receptor (HR) positive metastatic breast cancer (MBC), but little is known about its utility in patients with breast tumors that overexpress human epidermal growth factor 2 (HER2). This evidence concerns the gene ESR1 and breast neoplasm.