The goals of this review will be to highlight the historical evolution of Ki-67 in breast cancer, summarize the present literature, including the International Ki-67 in Breast Cancer Working Group (IKWG) efforts to standardize Ki-67 assessment [2,13], with an emphasis on the clinically-relevant literature, and to consider the future implications of the monarchE trial (which led to the U.S. approval of Ki-67 as a companion diagnostic biomarker in breast cancer) [26], and other recent developments in breast cancer Ki-67 research. The gene discussed is MKI67; the disease is breast carcinoma.