ESR1 and breast cancer: The hypothesis-driven approach described in Methods was applied to the training set and identified 10 markers in three functional groups known to be important to breast cancer behavior: ER function (ER, PR, Bcl2, GATA3, EIG121), tyrosine kinase receptor function (EGFR, HER2, HER2p1248), and cell proliferation (CCNB1, CCNE1).