These hormonal actions are mainly mediated by ERα and ER-β [18,19,20,21], which are strongly expressed in human breast cancer, ERβ being predominant in benign breast tumors, while ERα is dominant in malignant tumors [22,23,24], in such a way that ERα is associated with growth and cell proliferation in breast cancer [25,26]. Here, ESR2 is linked to breast cancer.