ESR1 and breast cancer: Mutations in the ligand-binding domain of ESR1 itself have been found in several cases of metastatic ER + BC after treatment with antiestrogen therapy (Robinson et al. 2013, Toy et al. 2013), but Fulvestrant is potentially effective in such ESR1-mutant cells (Li et al. 2013, Merenbakh-Lamin et al. 2013, Toy et al. 2013).