ESR1 and breast cancer: Achieving pathological complete response (pCR) after NAT is associated with favorable long-term outcomes, allowing guidance of further treatment.3, 4, 5 The response to NAT varies across molecular subtypes of breast cancer, primarily defined by estrogen receptor/progesterone receptor (ER/PR) and human epidermal growth factor receptor 2 (HER2) status, with triple-negative (HER2−&ER/PR−) and HER2+ tumors showing the highest pCR rates.6