ESR1 and breast cancer: Based on the receptors on the surface of breast cancer cells, such as the hormone receptors estrogen receptor (ER) and progesterone receptor (PR) and the human epidermal growth factor receptor 2 (HER2), four subtypes can be distinguished: Luminal A (Luminal A, ER+/PR+ or -/HER2-), Luminal B (Luminal A, ER+/PR+ or -/HER2-) and Luminal B (Luminal A, ER+/PR+ or -/HER2-).