ESR1 and breast cancer: In addition to ER, a number of other HSP90 client proteins have been shown to be involved in breast cancer progression such as those that are important for signaling through the phosphahtidylinositol-3-kinase (PI3K/p110α)/protein kinase B (PKB/AKT) pathway, including epidermal growth factor receptor (EGFR) 1 and 2 (ERBB2) and AKT [16,17].