ESR1 and breast cancer: The overexpression of PAK‐1 is believed to be responsible for the phosphorylation of the estrogen receptor, creating promiscuous phosphorylated receptors resistant to tamoxifen treatment.39, 40, 41 Our studies further support the link between ER expression and PAK‐1 expression in breast cancer cells and suggest a link between the efficacy of PAK‐1 inhibitors and ER status.