ERBB2 and breast cancer: In the US, breast cancer is the most commonly diagnosed female malignant neoplasm,1 with 70% of diagnoses being hormone receptor (HR) positive and ERBB2 (formerly HER2) negative.2 While many women with early-stage HR-positive, ERBB2-negative breast cancer do well with adjuvant endocrine therapy alone, some patients benefit from the addition of adjuvant chemotherapy to further reduce the risk of recurrence and improve survival outcomes.