ESR1 and breast cancer: To validate that the different ERα ligand concentrations were biologically relevant, we used this model to optimize our proliferation assays using estradiol (E2), the ERα-specific ligand PPT, the ERβ-specific ligand DPN, five distinct SERMs that show anti-estrogen activity in breast cancer cells, and fulvestrant, a pure anti-estrogen that show antagonist functions of both receptors in all tissues tested.