Although advances in therapeutics, such as cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, immune checkpoint inhibitors, and novel anti-human epidermal growth factor receptor 2 (HER2) antibody-drug conjugates (ADCs)have improved treatment options, distant metastases remain a major cause of treatment failure and mortality in breast cancer. The gene discussed is ERBB2; the disease is breast cancer.