ESR1 and breast carcinoma: ERα plays an important role in regulating mammary epithelial cell proliferation, differentiation and tumorigenesis.6 ERα can both activate and repress the expression of downstream target genes as a ligand-activated transcription factor.7 ERα does also serve as an important prognostic factor in breast cancer,8 and luminal subtype A tumors have the best clinical outcome among all subtypes.3