ESR1 and breast cancer: Patients with low ER levels (1% to 9%, ER-low) derive limited benefit from adjuvant ET (2-9), and yet share similar clinicopathological characteristics (3,4,10), prognosis (11,12), response rates to neoadjuvant chemotherapy (NACT) (11,13), and prognostic effect of pathological complete response (pCR) (14) as ER <1%/HER2-negative (ER-neg) BC.