ESR1 and breast cancer: The effect of P-I against breast cancer is: broad, encompassing both ER-positive and triple-negative human xenografts; strong, as evidenced by the tumor stasis and tumor regression achieved in MDA-MB231 and MCF-7 xenografts, respectively; and potentially clinically relevant, as these two cell lines represent the most frequent and the most difficult-to-treat subtypes of breast cancer.