CDK4 and breast cancer: Currently, selective ER modulators (SERM), selective ER degraders (SERD), aromatase inhibitors (AIS), ovarian function inhibitors (OFS), mTOR inhibitors, and cell Cyclin-dependent kinase 4/6 (CDK4/CDK6) inhibitors are the main endocrine and targeted therapies for HR+ breast cancer patients [5–7].