ESR1 and breast cancer: PAK‐1 expression is increased during the early stages of human breast cancer progression.12, 36 Studies suggest that PAK‐1 overexpression can predict tumor recurrence and resistance to tamoxifen, which is a selective estrogen receptor modulator commonly used for the treatment of hormone‐receptor‐positive, early stage breast cancer.37 Toward this hypothesis, the IC50 of IPA‐3 was highest in estrogen receptor (ER)‐positive breast cancer cells, MCF‐7 and BT‐474, while the IC50 was lower in ER‐negative cells (MDA‐231 and MD‐468).