CXCR4 and breast cancer: Estrogen also promotes the epithelial-mesenchymal transition and stemness of ER-positive breast cancer cells [52, 53] through hedgehog signaling activation [54], G protein-coupled estrogen receptor (GPCER) regulation [55], or crosstalk to Notch signaling [56], SDF-1/CXCR4 signaling [57] and TGFβ signaling [58].