16α-[18F]-fluoro-17β-estradiol (18F-FES) is an ER-specific imaging probe, and 18F-FES PET has been extensively performed for the noninvasive imaging and measurement in vivo of ER expression breast cancer because 18F-FES uptake correlates well with the immunohistochemical scoring for ER [23–25]. Here, ESR1 is linked to breast cancer.