The [18F]FES PET radiotracer has been usedin ER+ tumors for preclinical evaluation of a therapeuticresponse and clinical visualization of breast cancer with an averageof 90% specificity and 85% sensitivity.86−92 In the case of PR, [18F]FFNP was used in a clinical study,identifying ∼94% of PR+ breast tumors using PET.93 To visualize HER2+ breast tumors,both PET and SPECT radiopharmaceuticals based on the monoclonal antibodytrastuzumab were tested,14−18,94 and the efficacy of 64Cu-labeled trastuzumab to detect HER2-positive breast cancer wasconfirmed.95 Here, ESR1 is linked to breast carcinoma.