ESR1 and breast cancer: In addition, Fulvestrant, the main drug in the group of Selective estrogen receptor downregulators (SERDs) is used as an alternative in tamoxifen resistant breast cancers and acts by disrupting the ER receptor and blocking ER dimerization which in turn inhibits estrogen signaling via ER down-regulation (Osborne, Wakeling & Nicholson, 2004).