ESR1 and breast cancer: Our results indicate induced expression of PPM1A can strongly suppress cell growth of the PPM1A-transfected ER-negative cell lines tested (SUM159: 73% repression; MDA-MB-231: 59% repression), but does not suppress growth in PPM1A-transfected ER-positive (MCF7) breast cancer cells (Fig. 3c–e, respectively).