PPM1D and breast cancer: Depletion of miR-21 or WIP1 markedly decreased the growth of xenograft tumors (Fig. 6f, g and Supplementary Fig. 6c, d), and their dual depletion led to more severe tumor growth inhibition, indicating a synergistic role of miR-21 and WIP1 in the trastuzumab resistance of HER2+ breast cancer.