ESR1 and breast cancer: The physiological impact of FKBP4 on ER activity in FKBP4 knockout mice 48, 49, as well as its role in breast cancer, were not clearly demonstrated, despite: (i) its strong correlation with ER expression 17, 18; (ii) the upregulation of its gene expression by estrogen 19; and (iii), its association with ER/HSP90 complexes 20.