CDK4 and breast cancer: Breast cancer remains a leading cause of cancer morbidity and mortality, with hormone receptor–positive, ERBB2–negative (formerly HER2 or HER2/neu) metastatic breast cancer being the most common subtype.1,2,3,4 Cyclin-dependent kinase 4/6 inhibitor inhibitors (CDK4/6i) combined with endocrine therapy (ET) have established efficacy as first-line treatment for hormone receptor–positive/ERBB2–negative metastatic breast cancer,5 and their introduction results in improved progression-free survival (PFS) and overall survival (OS) compared with ET only.6,7,8,9,10,11,12,13,14