MKI67 and breast cancer: Prior research has categorized breast cancers into four principal subtypes based on gene expression profiles and the expression of molecular biological markers, including luminal A (positive for estrogen or progesterone receptor ER+/PR+ with high Ki67 expression), luminal B (ER+/PR+ with low Ki67 or ER+/PR+/HER2+), HER2+(positive for human epidermal growth factor receptor 2) and triple negative (absence of estrogen, progesterone and HER2 receptors) (2, 3).