ESR1 and breast neoplasm: Recent studies revealed that breast tumors acquire mutations in the ER ligand binding domains (L536N, Y537S, Y537N and D538G) that facilitate constitutive activity of these mutant ER (MT-ER) in the absence of ligand (Toy et al., 2013; Robinson et al., 2013; Jeselsohn et al., 2014; Merenbakh-Lamin et al., 2013).