CD44 and breast cancer: SRGN induced TGFβ2 (transforming growth factor-β2) expression by activating CD44/ CREB1 signaling, and TGFβ2 induced SRGN expression by activating Smad3 to form a positive feedback loop, which in turn promoted epithelial-to- mesenchymal transition (EMT) to enhance TN-BC metastasis 20.