ESR1 and breast cancer: Secondly, given the established association between ctDNA prevalence and breast cancer subtype, and the known prognostic variability across subtypes, to mitigate the potential confounding effect of subtype-associated prognosis, survival analysis was performed in a cohort of estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative and node-negative (ER+/HER2−/n0) invasive breast cancer patients (n = 120).