ESR1 and breast cancer: With growing understanding of the biology of breast cancer, a novel prognostic staging system (pathologic prognostic staging [PPS]) has been developed in the most recent, 8th edition of AJCC (American Joint Committee on Cancer) by incorporating histologic grade, estrogen receptor (ER), and human epidermal growth factor receptor-2 (HER2) status to the traditional TNM staging system [5].