ERBB2 and breast carcinoma: Hormone receptor positive (HR+) breast cancers represent approximately 70%-80% of all breast cancers seen among women.1,2 Metastatic HR+/human epidermal growth factor receptor 2-negative (Her2−) breast cancer has been historically treated with sequential courses of endocrine therapy (ET) prior to the eventual need for cytotoxic chemotherapy.3,4 Attempts to both extend and enhance the efficacy of ET have led to the discovery and development of pharmacological inhibitors of the cyclin-dependent kinases 4 and 6 (CDK 4/6).