ARID1A and breast cancer: In this study, luminal early BCs from the prospective WSG‐ADAPT trial which did not respond to short‐term preoperative endocrine therapy (pET) were analyzed for alterations of genes which had recently been implicated in endocrine resistance in metastatic BC including ARID1A, CCND1, ERBB2, ESR1, FGFR1, PAK1, PIK3CA, and TP53.7