ESR1 encodes estrogen receptor alpha (ERα), a nuclear receptor functioning as a ligand-dependent transcription factor in breast tissue. The receptor binds estrogen response elements (EREs) to regulate gene expression and activates endothelial nitric oxide synthase (NOS3) for nitric oxide production 1. ESR1 exists in multiple isoforms that modulate transcriptional activity through competitive binding and heterodimerization mechanisms 2. Clinically, ESR1 mutations represent a critical mechanism of endocrine therapy resistance in hormone receptor-positive metastatic breast cancer (MBC), occurring in up to 36% of patients 3. Common mutations include D538G (36% of cases) and Y537S (14%), which promote ligand-independent, constitutive receptor activation 4. These mutations are selectively enriched during aromatase inhibitor therapy, particularly in the metastatic setting (36.4% prevalence) compared to adjuvant exposure (5.8%) 5. ESR1-mutant tumors demonstrate substantially shorter progression-free survival on subsequent aromatase inhibitor therapy (hazard ratio 3.1) 5. ESR1 mutations can be detected in circulating tumor DNA using digital droplet PCR and next-generation sequencing 6. Recent clinical approvals demonstrate that selective estrogen receptor degraders (SERDs), particularly elacestrant, significantly improve progression-free survival in ESR1-mutant MBC (hazard ratio 0.55) 7, representing a therapeutic breakthrough for this endocrine-resistant phenotype.