ESR1 and breast cancer: However, they can be roughly classified using immunohistochemical (IHC) biomarkers, because there is a high correlation between the molecular subtypes and IHC results.[6–11] Luminal breast cancer is usually characterized by signal expression of receptors for estrogen receptor-positive (ER + ) or progesterone receptor-positive (PR + ), with either low proliferation of Ki67 index (A-type) or high proliferation of Ki67 index (B-type).