ESR1 and breast cancer: Figure 2B shows an example of 18F-FFNP PET images in a PR-positive and a PR-negative BC patient. Previous research reported a decrease in PR expression after successful anti-estrogen treatment as a result of inhibition of ER-activated pathways [28] and preclinical studies investigating the potential of 18F-FFNP PET imaging to predict response to anti-estrogen treatment have been performed with promising results [29,30].