ESR1 and breast cancer: The clinical benefit rate (CBR) at 6 months, composed of all complete responses (CR), PR and SD observed as best responses, was 21%, the median PFS were of 4.1 months (95% CI 2.3–7.7) for patients with ER+ HER2– BC, 18.8 months (95% CI: 5.1—NE) for ER+ HER2+ patients and 1.8 months (95% CI: 0.9—NE) for patients with triple negative (TN) tumors, respectively.