ESR1 and breast cancer: The ERs are expressed in 60-70 % of BCs (Lim et al., 2012[25]) and ERα seems to be the predominant form (Martin et al., 1991[28]); therefore, targeting of ER is a suitable strategy for management of ER-positive BCs, and antiestrogens like tamoxifen which are used for BC therapy act through ER targeting.