ESR1 and breast cancer: The main recognized breast cancer subtypes are as follows: Luminal A, estrogen-receptor (ER) positive, Ki-67 < 14%, and normal expression of Her2; Luminal B, ER-positive, Ki-67 ≥ 14%, and normal expression of Her2; Luminal/Her2+, ER-positive and Her2 overexpression; Her2-enriched, ER-negative and Her2 overexpression; and triple negative (TN), ER-negative, progesterone receptor (PR) negative, and normal expression of Her2.