Mortimer et al. [62] studied this metabolic effect with FDG-PET and 16 alpha-[29]fluoroestradiol-17 beta (FES)-PET in 40 women with biopsy-proved advanced ER-positive breast cancer receiving therapy with tamoxifen and concluded that this flare effect could be an indicator of hormone responsiveness in advanced breast cancer. Here, ESR1 is linked to breast cancer.