CDK4 and breast cancer: The use of CDK4/6 inhibitor therapy has gained ground since its approval in Europe in late 2016 (2) and the latest European Society for Medical Oncology (ESMO) consensus guidelines for advanced breast cancer describe a CDK4/6 inhibitor combined with endocrine therapy as “the standard-of-care first-line therapy” for patients with HR+/HER2 − metastatic breast cancer (3).