CDK4 and breast cancer: In recent years, new targeted drugs have emerged, such as (1) multi-targeted small-molecule tyrosine kinase inhibitors (TKI), e.g., lapatinib (16); (2) inhibitors of cyclin-dependent kinases 4/6 (CDK4/6), e.g., reboxetine (17); and (3) anti-vascular endothelial growth factor (VEGF) monoclonal antibodies, e.g., bevacizumab (18, 19), offering new options for the treatment of HER2-positive BC.