ESR1 and breast cancer: Moreover, shRNA-mediated SNAI2 knockdown halted the hypoxia-induced down-regulation of the epithelial differentiation markers estrogen receptor alpha (ESR1), keratin 18 (KRT18) and e-cadherin (CDH1) (Figure 1E and Figure S1C) and the hypoxia-induced up-regulation of CD44 expression (Figure 1F), a marker of breast cancer stem/progenitor cells [21,40].