ESR1 and breast cancer: TNBC, including basal-like and claudin-low subtypes, accounts for 10–20% of breast cancers and is characterized by a lack of PR, ER and HER2 overexpression.69 TNBC patients present with higher incidence of distant disease recurrence within 3 years of diagnosis, with a high frequency of visceral metastases.70 The prognosis for patients diagnosed with TNBC is poor, with patients who respond poorly to adjuvant treatment exhibiting worse outcomes.4