ESR1 and breast cancer: We first conducted an examination of PPM1A expression across several publicly available breast cancer microarray datasets.16,24–30 Details of these datasets are described in “Methods” and are listed in Mazumdar et al.31 As shown in Fig. 1a, PPM1A is underexpressed in ER-negative tumors as compared to ER-positive tumors in eight individual human breast cancer data sets.