ESR1 and breast cancer: Currently, five molecular types of BC are distinguished: luminal type A (ER + , PR + , HER2-, low Ki67), HER2- luminal type B (ER + , low PR or high Ki67, HER2-), luminal B/HER2 + (ER + and HER2 +), HER2-positive non-luminal (ER-, PR-, HER2 +).