CDK4 and breast cancer: On the other hand, Ki-67 has struggled to join this established trifecta in the routine management and risk stratification of breast cancer patients [1–4] and its prognostic and predictive value is restricted to very specific settings in breast cancer; recently, the results from the monarchE study has suggested a prognostic role for Ki-67 ≥ 20% in patients with early breast cancer treated with cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors in combination with endocrine therapy [5].