ESR1 and breast cancer: The most prevalent subtype of BC was hormone receptor-positive (either estrogen receptor or progesterone receptor) and HER2 negative: (HR+)/HER2- (58%, n = 583, 113), followed by HR+/HER2+ (26%, n = 258,750), HR-/HER2-negative (10%, n = 104,175), and HR-/HER2+ (6.4%, n = 64, 947).