ESR1 and breast cancer: Various PET and single-photon emission computer tomography (SPECT) tracers labeled with, e.g., F-18 [6,7,8,9], Tc-99m [10,11] or I-131 [12,13] as a radionuclide are reported in the literature, but so far, 16α-[18F]fluoroestradiol (FES) is the only FDA-approved imaging probe for PET imaging of ER+ breast cancer [14].