In 2021, a Ki67 immunohistochemistry (IHC) assay with an LI cut-off of 20% was approved by the Food and Drug Administration as a companion diagnostic for combination with abemaciclib, a cyclin-dependent kinase 4/6 inhibitor, and endocrine therapy in hormone receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative, node-positive, early breast cancer [7]. The gene discussed is ERBB2; the disease is breast cancer.