CDK4 and breast cancer: Endocrine therapies for breast cancer can be divided into several categories based on the production pathway and role of the hormone receptors in the occurrence and progression of breast cancer (Table 2), including antiestrogen drugs (selective ER regulator, selective ER downregulation), aromatase inhibitors, luteinizing hormone releasing hormone analogues (LHRH analogues), progesterone drugs, and CDK4/6 inhibitors.