ERBB2 and breast cancer: Palbociclib, an oral inhibitor of cyclin-dependent kinase 4/6 (CDK4/6), is indicated in combination with an aromatase inhibitor as first-line treatment for hormone receptor–positive, human epidermal growth factor receptor 2–negative advanced breast cancer (HR+/HER2− ABC) and in combination with fulvestrant in women with HR+/HER2− ABC whose disease progressed on prior endocrine therapy (ET) [1, 2].