The global prevalence of breast cancer has substantially increased over recent decades.1 In South Korea, breast cancer is one of the most common malignancies among females, accounting for >2000 deaths in 2016.2 Since breast cancer is a heterogenic disease with several biological characteristics, various factors such as hormone receptors, human epidermal growth factor receptor 2 (HER2), grade and Ki-67 labelling index (LI) affect disease prognosis.3,4 Currently, significant efforts are being made to develop methods for the accurate prediction of breast cancer prognosis. The gene discussed is MKI67; the disease is breast cancer.