The recent FDA approval of abemaciclib (based on the seminal monarchE trial [26]), a cyclin-dependent kinase (CDK) 4/6 inhibitor, as an adjuvant treatment option for high-risk ER-positive/human epidermal growth factor receptor-2 (HER2)-negative early stage breast cancer, relies on a quantitative assessment of Ki-67 expression in the treatment decision algorithm. The gene discussed is MKI67; the disease is breast cancer.