ESR1 and breast cancer: The involvement of the estrogen receptor α (ER) in the development and progression of breast cancer (BC) has been confirmed by numerous ‘in vivo’ and ‘in vitro’ studies that have elucidated many aspects of ER regulation and functions, paving the way to the wide range of endocrine therapies commonly used in clinics for the treatment of ER-positive (ER+) BC patients [1,2,3].