PTEN and breast cancer: Importantly, in BC cells, miR-21 regulates EMT through inhibition of PTEN function via p-AKT and p-ERK pathways, and re-expression of miR-21 leads to the acquisition of EMT phenotype in bCSCs with the activation of mesenchymal markers (vimentin, N-cadherin, α-SMA) (219, 220).