ESR1 and breast cancer: Five molecularbiological subtypes of breast cancer are recognized today: ER+ luminal A breastcancer (HER2-negative, low Ki-67 expression (≤ 20%), and highprogesterone receptor (PR) level (≥ 20%)); HER2-negative luminal B breastcancer: ER+, HER2-, one of the following factors is present: high Ki-67expression (≥ 30%) or low PR level ( < 20%); HER2-positive luminal Bbreast cancer: ER-positive, HER2-positive, any level of Ki-67 expression, anyPR level; HER2+: HER2+, ER- and PR-, any level of Ki-67 expression; and triplenegative breast cancer (TNBC): ER-, PR-, HER2- [4].