Palbociclib was the first cyclin-dependent kinase 4/6 (CDK4/6) inhibitor to receive accelerated approval by the United States (US) Food Drug Administration (FDA) in February 2015 to treat post-menopausal women for advanced stage Hormone Receptor Positive (HR+)/Human Epidermal Growth Factor Negative (HER2-) breast cancer in combination with letrozole as initial endocrine therapy [1]. This evidence concerns the gene ERBB2 and breast cancer.