CDK4 and breast cancer: This review comprehensively summarizes recent advances in targeted therapies for major BC subtypes: endocrine therapy combined with cyclin-dependent kinase 4/6 (CDK4/6) or phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) inhibitors for HR-positive BC; novel antibody‒drug conjugates (ADCs) such as trastuzumab deruxtecan (T-DXd) and tyrosine kinase inhibitors (TKIs) for HER2-positive BC; and trophoblast cell-surface antigen 2 (Trop-2) ADCs, immunotherapies, and poly-ADP-ribose polymerase (PARP) inhibitors for TNBC.