ERBB2 and breast cancer: The development and approval of cyclin-dependent kinase 4 and 6 (CDK4 & 6) inhibitors for hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) advanced breast cancer (ABC) has changed the treatment paradigm of this disease by significantly extending progression-free survival (PFS) and overall survival (OS)1–4.