Washington – October 16, 2026 -- Nearly half of patients with hormone receptor positive, HER2 negative advanced breast cancer develop resistance to first-line endocrine therapy driven by the ESR1 mutation (ESR1m), according to a new Academy of Managed Care Pharmacy (AMCP) Market Insights Program report. The report finds that biomarker testing and treatment paradigms are evolving faster than the evidence payers need to manage them.