MKI67 and breast cancer: Briefly, breast cancer patients can be classified as multiple molecular subtypes (e.g., luminal A (ER+/PR+ and Ki67-low), luminal B (ER+/PR+ and HER2+ or HER2–, and Ki67-high), HER2-enriched (HER2+), normal breast-like, basal-like, and claudin-low) [30].