MKI67 and breast cancer: Luminal-type (hormone receptor–positive) breast cancer is the most frequent subtype and is itself divided into two subtypes, namely Luminal A and Luminal B. Luminal A is defined as a low-proliferation subtype (Ki-67 < 14), whereas Luminal B is defined as a high-proliferation subtype (Ki-67 ≥ 14) [2,3,4].