MKI67 and breast cancer: In particular, the two luminal subtypes are HR+, differing from each other in the expression of HER2 and the Ki-67 score; indeed, luminal A tumors (50–60% of all breast cancer cases) are HER2- with a Ki-67 score < 20%, whilst most luminal B tumors are HER2+ with a Ki-67 score ≥ 20%.