MKI67 and cancer: In detail, Luminal A-like tumors, characterized by high PR (i.e. ≥ 20%) and low Ki67 (i.e. <20%) levels, have an excellent prognosis and endocrine sensitivity, while luminal B-like HER2 negative cancers, identified by low PR (i.e. < 20%) and/or high Ki67 (i.e. ≥ 20%) values, represent an extremely heterogeneous subgroup associated with a slightly unfavorable outcome (4).