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. Here, ERBB2 is linked to cancer.