ESR1 and breast cancer: Breast cancer (BC) is the most prevalent cancer, with a wide spectrum of clinical and pathological presentations.1 One of the most important features to characterize BC is estrogen receptor (ER) status, according to which BC is categorized into ER positive and ER negative.2 Patients with ER-negative BC are typically younger, insensitive to hormonal therapy, and have a lower 5-year survival of 78% compared with 89% in patients with ER-positive BC.