ESR1 and metastatic neoplasm: Beyond FDG, other U.S. Food and Drug Administration (FDA) approved radiopharmaceuticals for breast cancer imaging, at time of this writing, are 18F-fluoroestradiol (FES), which binds to estrogen receptors (ERs) for assessment of ER+ breast malignancy, and 18F-sodium fluoride (NaF), that allows assessment of osseous metastatic disease for breast and other cancers [67,68,69].