ESR1 and invasive ductal breast carcinoma: The example is of a common type of ER+ breast cancer, i.e., a 55-year-old patient with 2.0 cm, grade II (Nottingham score 6, with mitosis score of 1), lymph node negative, ER+ (H-score of 300), PR negative (H-score 0), HER2 negative, Ki-67 labeling index of 15% invasive ductal carcinoma (Fig. 3).