MiR-21 is overexpressed in HER2+ (human epidermal growth factor receptor 2-positive) and triple-negative breast cancer and inhibits autophagy by targeting PTEN, a negative regulator of P13K activity by dephosphorylation of P1P3 (phosphatidylinositol 3,4,5-triphosphate) [85,86,87], thereby mediating anti-apoptotic and chemoresistance responses in breast cancer. This evidence concerns the gene PTEN and breast cancer.