ESR1 and breast cancer: We have previously established and characterized subclones of the human BC cell line MCF-7 expressing human ERβ fused to a Tandem Affinity Purification (TAP) tag, and have shown that the addition of the TAP-tag at either the N- or the C-terminus of the protein (indicated as Nt-ERβ and Ct-ERβ, respectively) does not alter significantly the receptor function, nor its ability to activate transcription or to antagonize ERα-dependent transcription [21,22,36].