ESR1 and breast carcinoma: Clinical evidence from the CheckMate 7FL and KEYNOTE-756 trials shows that successful conversion in HR+ breast cancer correlates with specific baseline features: PD-L1 positivity, with pCR rates over 40% using checkpoint blockade versus 20% with chemotherapy alone; lower ER expression, with tumors expressing less than 10% ER benefiting more; and baseline TILs presence [6,7,111].