ESR2 (estrogen Receptor β) displays a consistent homology with ESR1 (Estrogen Receptor α); in spite of this structural homology, ESR2 displays a pattern of expression and biologic functions different from ESR1. However, the exact function of ESR2 in breast cancer remains unclear, since some studies suggested a proliferative and other one an anti-proliferative effect on these tumor cells [331]. This evidence concerns the gene ESR2 and neoplasm.