CDK4 and breast cancer: As for HR+ breast cancer treatment, endocrine therapy remains a cornerstone, but improvements in patient outcomes have been achieved by introducing novel therapeutic agents that act on extra-hormonal molecular targets—such as cyclin-dependent kinase 4/6 (CDK4/6), the mammalian target of rapamycin (mTOR), protein kinase B (also known as Akt), and phosphatidylinositol 3-kinase (PI3K).