PGR and breast cancer: Therefore, based on the expression of ER, PR, HER2 and Ki-67 detected by immunohistochemistry (IHC), breast cancer is classified into five groups: luminal A (ER and PR-positive/HER2 negative/Ki-67 ‘low’), HER2-negative luminal B (ER positive/HER2-negative/Ki-67 ‘high’ or PR ‘negative or low’), HER2-positive luminal B (ER-positive/HER2 overexpression/any Ki-67 or PR), HER2-positive (HER2 overexpression/ER and PR absent) and triple negative (ER and PR absent/HER2-negative) [21].