And Snail can also inhibit the expression of other epithelial genes such as Claudin1 and Muc1 and promote the expression of other mesenchymal genes such as fibronectin, MMP9 and vimentin, which activates EMT and is related to tumor metastasis, recurrence and poor prognosis in breast cancer [33–35]. The gene discussed is MMP9; the disease is breast cancer.