CDK4 and breast cancer: Over the last 15 years, breast cancer treatment has shifted to a more personalized treatment, based on targeted therapies (hormone receptor-based, HER2 status-based, BRCA mutation-targeted, and CDK4/6 inhibitors) and immunotherapy (PD-1 inhibitor, antibody–drug conjugate T-DM1) [25–27], bolstered by novel diagnostic techniques, such as gene expression profiling [28].