CDK4 and breast cancer: Palbociclib and ribociclib are breakthroughs in the treatment of advanced HR+/HER2− postmenopausal breast cancer, overturning the old pattern of single endocrine therapy for advanced postmenopausal breast cancer in the past decades, laying the foundation of CDK4/6 inhibitors combined with aromatase inhibitors in first-line treatment for postmenopausal women with HR+/HER2− advanced breast cancer.