ESR1 and breast cancer: As laso and RAD1901 are under clinical evaluation in Y537S ESR1 breast cancer patients6,38, we measured the anti-proliferative activities of T6I-29-1A alongside 4OHT, ICI, laso, and RAD1901 at 1 μM in homozygous Y537S ESR1 MCF7 cells (kindly donated by Dr. Sarat Chandarlapaty, Fig. 8c).