ESR1 and breast cancer: We used the long established ER+ breast cancer cell line MCF-7 and measured the effect of recombinant human Notch-receptor ligand DLL4 (rhDLL4) on the proportion of BC cells characterized by stem-cell phenotype CD44+CD24-/low[1], [7], and on MS formation, as a measure of SC number; [7], [18] MS are colonies formed in suspension from single cells and therefore measure clonogenic potential; cf. ref. [7]).