ESR1 and breast cancer: The modern classification of lobular breast cancer (LBC) includes five subtypes of different molecular profile: luminal A (ER+ and/or PR+, HER2−, Ki67<15%), luminal B (HER2− subtype: ER+ and/or PR+, HER2−, Ki67≥15%; HER2+ subtype: ER+ and/or PR+, HER2+), HER2 type (ER− and PR−, HER2+), and triple negative (TN) breast cancer (7).