ESR1 and breast carcinoma: Building on these results, the BOLERO-4 clinical trial (NCT01698918) (Study Details Open-label, Phase II, Study of Everolimus Plus Letrozole in Postmenopausal Women With ER+, HER2- Metastatic or Locally Advanced Breast Cancer ClinicalTrials.gov, 2023) examined the efficacy and safety of first line everolimus and letrozole in postmenopausal patients with ER+/HER2-metastatic or locally advanced breast cancer.