ESR1 and cancer: Collectively, ER+ cancers are classified as luminal, which are further subclassified based on their HER2 status and proliferation rate as Luminal A (ER/PR+, HER2−, Ki67+ < 20%), Luminal B (ER/PR+ < 20%, HER2−, Ki67+ ≥ 20%), and triple positive HER2+ B2 (ER/PR+, HER2 overexpression).