ESR1 and breast cancer: Moreover, in a mouse model of subcutaneous MCF7-Y537S mutant ERα + breast cancer, 18F-fluoroestradiol positron emission tomography (18F-FES PET) imaging confirmed that amcenestrant dose-dependently inhibited tumor uptake of 18F-FES, which correlated with immunohistochemical scoring for ERα expression, and 18F-fluorothymidine (FLT) PET showed a significant decrease in tumoral FLT accumulation when amcenestrant was combined with palbociclib34.