A study in 2022 of 7 metastatic estrogen receptor positive/progesterone receptor positive/human epidermal growth factor 2 negative (ER+/PR+/HER2-) breast cancer patients using 64Cu-SAR-BBN PET/CT evaluated the safety and efficacy of this agent, as well as comparatively evaluated its ability for lesion characterization against 18F-FDG. This evidence concerns the gene ESR1 and breast cancer.