ESR1 and neoplasm: Our results support a new testable breast CSC model (Fig 6B) that is consistent with our hypothesis in that the postulated small subpopulation of CSCs resides in the intermediate hybrid E/M state, independent of the tumor and cell-line context (luminal epithelial and basal mesenchymal breast cell lines and luminal ER+ and basal ER- tumors): stem-like hybrid E/M cells can arise in both the E and M cell populations through incomplete EMT and MET, respectively.