ERBB2 and breast cancer: Breast cancer is the most frequent malignant neoplasm observed in women and is the leading cause of cancer death.1 The most frequent subtype of breast cancer is hormone receptor (HR)–positive, ERBB2 (formerly HER2)–negative breast cancer, which accounts for approximately 65% of all breast cancers.2,3 Nearly two-thirds of patients with metastatic breast cancer are HR–positive, and it is estimated that about 25% of patients with breast cancer will experience recurrence after surgery.4,5