ESR1 and breast cancer: A large body of experimental and clinical evidence has determined that loss of PTEN promotes tamoxifen resistance in breast cancer.[26, 27] Moreover, loss of PTEN in ERα‐positive breast cancer is predictive of reduced recurrence‐free survival after tamoxifen.[28] Western blotting assays revealed that BCs transfected with miR‐22 mimics showed lower PTEN expression than control BCs (Figure 5E).