The OP-1250 clinical trial (NCT04505826) (Study Details A Dose Escalation/Expansion Study of Oral OP-1250 in Subjects With Advanced and/or Metastatic HR+, HER2- Breast Cancer ClinicalTrials.gov, 2023)evaluated the safety and efficacy of OP-1250 in patients with recurrent, locally advanced, or metastatic ER+/HER2-breast cancer that had progressed after previous endocrine therapy. Here, ESR1 is linked to breast cancer.