Systemic therapy for non-metastatic breast cancer is determined by subtype: patients with hormone receptor–positive tumours receive endocrine therapy, and a minority receive chemotherapy as well; patients with Erb-B2 receptor tyrosine kinase 2 (ERBB2)-positive tumours receive ERBB2-targeted antibody or small-molecule inhibitor therapy combined with chemotherapy; and patients with triple-negative tumours receive chemotherapy alone. This evidence concerns the gene ERBB2 and breast cancer.